Lithology and Origin of 

Middle Ordovician Calcareous 

Mudmound at Meiklejohn Peak, 

Southern Nevada 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 871 

Lithology and Origin of 

Middle Ordovician Calcareous 

Mudmound at Meiklejohn Peak, 

Southern Nevada 

By REUBEN JAMES ROSS, JR., VALDAR JAANUSSON, and IRVING FRIEDMAN 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 871 

A description of the various carbonate sediments constituting the 

Stromatactis-rich mound core and the basal "zebra limestone) n 

isotopic analyses of contrasting components) and review of possible 

complex origins of components 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1975 
UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress Cataloging in Publication Data 

Ross, Reuben James, 1918-

Lithology and origin of Middle Ordovician calcareous mudmound at Meiklejohn Peak, southern Nevada. 

(Geological Survey Professional Paper 871) 

Supt. of Docs. no.: I 19.16:871 

Bibliography: p. 

1. Rocks, Carbonate. 2. Geology, Stratigraphic-Ordovician. 3. Geology-Nevada-Meiklejohn Peak. 

I. Jaanusson, Valdar. II. Friedman, Irving, 1920- Ill. Title. IV. Series: United States Geological Survey Professional 

Paper 871. 

QE71.15.C3R67 5 51. 7'31 74-28131 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 

Stock Number 024-001-02690-2 
CONTENTS 

Page 

Abstract ____________________ ------_---__ 1 

Introduction _________________ ---____ ---- -

Purpose of report ______________________ - -

Acknowledgments ___________ ------ _ -----

Lithology-Continued 

Massive core facies of the carbonate mudmound, 

by Valdar Jaanusson-Continued 

Microscopic constituents-Continued 

General description and geologic setting of the mudmound 

1 

2 

2 

6 

7 

Composition of skeletal sand __________ -- Lithology ______________________________ _ 

Laminated limestone (zebra limestone), Comparison with selected other Paleozoic carbonate 

mounds __ ----------------------- by Reuben James Ross, Jr. ------------ __ Geologic relations ____________________ _ 7 

7 

7 

Isotopic interpretations, by Irving Friedman _________ _ 

Calcilutite layers ------------------- __ Origin of the mudmound -------------------- __ 

Radiaxial fibrous calcite layers ____________ _ 12 

15 

Origin of the zebra limestone, by Reuben James Ross, Jr. 

Geochemical evidence _________________ - Trace-element analysis -- _ --- ___ --- _____ _ 

Insoluble residue ------- __ - __________ _ 15 

Underlying limestone --------------------- 17 

Massive core facies of the carbonate mudmound, 

Physical evidence ____________________ _ 

Zebra limestone above base of mound _______ _ 

Pelletoid calcilutite ___________________ _ 

by Valdar Jaanusson ------------------ 20 

Aggrading neomorphism: Other views _______ _ 

Evidence of rupture structures ____________ _ 

Macroscopic constituents - --------------- 20 

Stromatactis ---------------------- 20 

Rapid submarine lithification _____________ _ 

Derivation from algal mats ------------- __ Macroscopic skeletal particles - - _____ - - - - _ 22 Effect of organic substances ______________ _ 

Fine-grained limestone ____ - - _____ - - - - _ 23 Theory of lifting by crystal growth _ _ _ _ _ _ _ _ _ _ _ 

Microscopic constituents ----------------- 24 

Main microscopic constituents ------------ 25 

Sparry calcite --------------------- 26 

Conclusions _______________ - ______ --

Origin of the core of the mudmound, by Valdar Jaanusson 

References cited __________________________ _ 

ILLUSTRATIONS 

Page 

FIGURE 1. Map of Nevada ____________________________________________________ -- _ 2 

2. Photograph showing carbonate mudmound from the north _________________________ ---- 3 

3. Photograph showing Meiklejohn Peak and the carbonate mudmound from the west __________ ---- 4 

4. Diagram showing approximate positions of collections on mudmound ____________________ --- 5 

5-34. Photographs: 

5. Cyclically laminated zebra limestone _______________________________ - --- 8 

6. Zebra limestone from flank of mudmound; polished section ___________________ --- 9 

7. Cyclic zebra limestone; large thin section ________________ --- ____________ -- 10 

8. One cycle of zebra limestone; thin section -------------------------------- 10 

9. Pelletoid calcilutite within radiaxial fibrous calcite __________________________ _ 10 

10. Disrupted layers of zebra limestone; thin sections ___________________________ _ 11 

11. Cyclic succession of three calcilutites, one partly converted to radiaxial calcite; thin section -- _ 12 

12. Pelletoid calcilutite appears cross bedded between disrupted laminae; polished surface _____ _ 12 

13. Calcilutite caa being converted to radiaxial fibrous calcite; thin section ____________ -- _ 13 

14. Radiaxial fibrous calcite and distribution of insoluble minerals in calcilutites; polished sections _ 14 

15. Zebra limestone, three cycles; thin section _______________ --- ___ ----------- 15 

16. Pelletoid crossbeds grading to radiaxial calcite; collodion peel and polished section _______ _ 16 

17. Radiaxial fibrous calcite associated with pelletoid calcilutite; collodion peel ___________ _ 16 

18. Incomplete conversion of calcilutite caa to radiaxial calcite; polished sections ___________ _ 17 

19. Detail of "cabbagehead"; thin section----------------------------------- 18 

20. Zebra limestone associated with "cabbageheads" ___________________________ _ 18 

21. Nautiloid chambers filled with para-axial calcite; shells surrounded by calcilutite and radiaxial 

fibrous calcite _____________________________________________ _ 19 

22. Poorly laminated limestone below mudmound; polished section ___________________ _ 20 

m 

page 

28 

29 

30 

33 

33 

34 

35 

38 

39 

40 

40 

41 

41 

42 

43 

43 

45 

46 
IV CONTENTS 

Page 

FIGURE 23. Poorly laminated limestone below mudmound; polished and thin sections _ _ _ _ _ _ _ _ _ _ _ _ _ _ 21 

24. Coarse calcarenite below mudmound _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 22 

25. Pelletoid texture, poorly laminated limestone; vertical and horizontal thin sections _ _ _ _ _ _ _ _ 23 

26. Sponge roots; polished section ___ ___ _ __ _ _ _ _ _ ____ _ _ __ _ _ _ ___ _ _ _ _ _ _ ___ _ _ 24 

27. Calcite-filled tubes in calcilutite; thin section _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 24 

28. "Microstromatactis" in mound core; thin section _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 25 

29. Echinoderm plate partly converted to radiaxial calcite; thin section _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 25 

30. Fossils relict in radiaxial calcite; thin section _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 26 

31. Ostracodes recrystallized into radiaxial calcite; thin section _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 26 

32. Two generations of limestone in mound core; thin section _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 26 

33. Faulted "microstromatactis"; thin section _____________________________ - _ _ 27 

34. Spiculaelike skeletal grains in mound core; thin sections ______________ - ____ - _- _ - 29 

35. Frequency composition diagram for oC13 ------------------------------------- ___ 33 

36. Frequency composition diagram for o018 ------------------------------------- ___ 34 

37-42. Photographs: 

37. Radiaxial calcite in contact with calcilutite caa that is partly converted to pelletoid calcilutite; 

polished sections _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 36 

38. Relationship of radiaxial calcite, pelletoid and equigranular caa between ca1 layers; collodion peel 37 

39. Radiaxial fibrm,1s calcite grading into pelletoid caa; collodion peel _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 37 

40. Cyclic calcilutites of zebra limestone; polished section _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 38 

41. Cyclic calcilutites and radiaxial fibrous calcite; polished section _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 39 

42. Filaments of Sphaerocodium; thin section ______________________ ------- _ __ 42 

TABLES 

Page 

TABLE 1. Relative proportions of microscopic constituents of mudmound -------------------------- 27 

2. Relative proportions of organic components of skeletal sand _____________________ ---_--- 28 

3. Isotopic analysis of limestone beds covering the mudmound ________________ - _- _---- _--- 31 

4. Isotopic analysis of zebra limestone and core of mudmound ___________ - _- _----_-------- 32 
LITHOLOGY AND ORIGIN 

OF MIDDLE ORDOVICIAN CALCAREOUS MUDMOUND 

AT MEIKLEJOHN PEAK, SOUTHERN NEVADA 

By REUBEN JAMES Ross, JR., 

VALDAR JAANussoN 1

, and IRviNG FruEDMAN 

ABSTRACT 

The large dome-shaped (300m wide by 80 m high) calcareous mudmound at Meiklejohn Peak is the largest and most accessible of three 

such Ordovician mudmounds in southern Nevada. It has been studied 

in hopes of deciphering the origin of zebra limestone which forms its 

base and of Stromatactis which is abundant in the main, virtually unstratified, core of the mound. The mudmound is completely enclosed 

within the Middle Ordovician Antelope Valley Limestone; it is underlain by calcarenite and poorly laminated limestone of the Paiute 

Ridge Member. Silty limestone of the Ranger Mountains Member intertongues with the lower third of the mound and abuts against and 

completely covers its higher parts. 

Cyclic laminated zebra limestone that forms the lower part of the 

mudmound is characterized by repeated sequences of three, or two, 

distinctive calcilutites. Radiaxial fibrous calcite separates some cycles; 

in these the third calcilutite tends to be missing. The origin of zebra 

limestone may be variously attributed to formation of submarine 

hardgrounds, to parallel shear cracks, to displacive precipitation, or to 

the precipitation of radiaxial fibrous calcite in parallel cavities of incredible extent. Some radiaxial calcite may have filled cavities but 

most resulted from recrystallization or neomorphic aggradation of 

other preexisting metastable carbonate. 

It seems likely that origin of the zebra limestone was linked to algal 

mats in light of geologic and geochemical evidence. 

The core of the mound is composed of 40-60 percent calcareous mud, 

20-60 percent sparry calcite, and 4-11 percent skeletal sand. 

Stromatactis is a prominent constituent of the sparry calcite. 

Regardless of the origin of Stromatactis as such, the radiaxial fibrous 

calcite crystals are the result of diagenetic change. The calcareous mud 

of the mound comprises at least two and possibly three generations. 

Within skeletal sand, echinoderms predominate; spicules, possibly of 

sponges, are second in abundance. Ostracodes outnumber trilobites. 

Neither bryozoans nor any other organism seems to have been present 

in numbers adequate to bind the mound sediments together. 

Stromatactis seems to represent cavity fillings. The radiaxial mosaic 

of Stromatactis was fully developed before the mound had ceased to 

grow. This indicates an early lithification of the mound. 

Examination of isotopes of oxygen and carbon indicates that 

calcilutites and radiaxial fibrous calcite formed in the same seawater. 

No evidence for fresh vadose water is present. 

'Naturhistoriska Riksmuseet, Stockholm, Sweden. 

INTRODUCTION 

Three Middle Ordovician carbonate mudmounds are 

known in southern Nevada (localities shown in fig. 1, this 

report; Ross and Cornwall, 1961; Ross, 1972, fig. 1). 

These mounds resemble somewhat younger Ordovician 

features in the Siljan district, central Sweden 

(Thorslund, 1936; Thorslund and Jaanusson, 1960, p. 24-

35), and Carboniferous "knoll reefs" of northwest and 

west-central Ireland ( Schwarzacher, 1961; Lees, 1964). 

Development of sparry calcite in the main part of the 

biggest of the Nevada mudmounds may have some relation to structures called Stromatactis, well known in 

middle and upper Paleozoic mudmounds in Western 

Europe (Black, 1952; Bathurst, 1959; Schwarzacher, 

1961; Lees, 1964; Jaanusson, 1975). In the basal part of 

the Nevada mound there is enigmatic, thinly interlayered calcilutite and sparry calcite which together 

resembles "sheet spar" described by Lees (1964, p. 523-

524) and "zebra limestone" described by Fischer (1964, 

p. 115, figs. 15, 17, and 18). 

At least three generations of sparry calcite are present. 

In the early phase of development there were two 

generations, one producing Stromatactis and the other 

producing the laminar sparry calcite near· the base. 

Limestone boulders, some of prodigious size, are 

known in southern Quebec (Mystic conglomerate) 

(Cooper, 1956, p. 14, 15, 31) and at Lower Head, western 

Newfoundland (Whittington, 1963, p. 7). Although no 

Stromatactis has been reported in them, some of these 

boulders are composed of limestone seemingly very 

similar to that in the main body of the Nevada mudmounds and the fossils obtained from them are so 

similar that their temporal correlation seems obvious 

(Cooper, 1956, p. 15, 31; Whittington, 1963; Ross, 1967, 

1970, 1972). Fossils from beds surrounding these 

boulders in Newfoundland corroborate the correlation 

with beds covering the Nevada mudmounds. 

1 
2 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

120° 118° 116° 114° 

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FIGURE 1.- Map of Nevada showing location of Nevada Test 

Site. Coevally formed Middle Ordovician mudmounds are 

known at Meiklejohn Peak and near Aysees Peak. 

The largest and most readily accessible Nevada mudmound is on the west face of Meiklejohn Peak, Bare 

Mountain quadrangle, 6 miles east of the town of Beatty, 

Nev. (fig. 2; Ross, 1972, fig. 2). 

Cephalopods from this mound have been studied by R. 

H. Flower; a stratigraphic resume of the fossils of the 

mound and covering beds, as well as descriptions of 

brachiopods and trilobites, was presented by Ross (1967, 

1972). Krause (1972) studied the inarticulate 

brachiopods of the mudmound and the covering beds; he 

concluded that infaunallingulides preferred the covering 

beds, whereas abundant acrotretids probably were attached to algae on the mound. The numerous complete 

bivalved acrotretids indicate that the upper part of the 

mound was deposited in relatively quiet water. A 

Master's thesis by Elizabeth Stilphen Yancey (1971), 

mentioned this locality. 

PURPOSE OF REPORT 

It is our purpose to describe the calcareous mudmound 

at Meiklejohn Peak in more detail than has been 

attempted previously (Ross, 1972), to compare it with 

similar structures in Europe with which Jaanusson has 

the greater familiarity, and to describe and speculate 

about the origins of various kinds of sparry calcite and 

about origins of the mound itself. 

The origin of the carbonate mudmound at Meiklejohn 

Peak has been enigmatic ever since the mound was first 

reported (Ross and Cornwall, 1961). Though it constitutes only a small part of the total volume, the origin 

of the basal zebra limestone facies has been particularly 

puzzling. 

Most colleagues knowledgeable in the study of carbonates with whom we have discussed possible origins 

have urged us to consider the high proportion of radiaxial fibrous calcite as the filling of former cavities. But 

all those who have accompanied us to the mound have 

agreed that, geologically and mechanically, cavities 

amounting to 60 percent of the volume of the basal facies 

are unreasonable and that the spar must have preferentially replaced some other material. 

It is our belief that the kinds of deformation shown by 

the laminated zebra limestone must have taken place 

prior to lithification and that many of these reflect 

earlier topography over which consecutive parallel 

laminae have been draped. 

In the main body of the mound Stromatactis accounts 

for about 20 percent of the volume. Some of it may have 

filled cavities but at least part replaced preexisting carbonate mud. 

ACKNOWLEDGMENTS 

In examining the mudmound in the field, we had the 

assistance of N. F. Sohl in 1969, W. T. Dean and Ellis 

Yochelson in 1970, and A. J. Rowell and F. F. Krause in 

1972, each of whom gave important suggestions about 

field observations and their interpretation. In particular, 

Rowell and Krause pointed out that mound sedimentation, whether along the base or above interfingerings of 

flank lithology high on the sides, is initiated by the 

laminated facies. Our conversations with J. L. Wray, 

Richard Rezak, L. A. Hardie, R. N. Ginsburg, P. R. 

Rose, and M. J. Brady have been concerned mainly with 

possible origins of the mudmound and expectable effect 

of algae on sedimentation. We are indebted to R. E. 

Wilcox for his petrographic examination of minerals in 

the insoluble residues from the laminated facies of the 

mudmound and to B. F. Leonard III and W. N. Sharp for 

X-ray diffraction identification of the same residues. 

Leonard and D. M. Pinckney have discussed replacement fabrics in carbonate rocks with us; 0. B. Raup, R. 

J. Hite, and W. C. Culbertson called our attention to 

similarities in lamination of certain saline deposits and 

the basal facies of the mudmound. A. T. Myers made a 

spectrographic analysis of the laminated limestone on 

which R. C. Surdam of the University of Wyoming ran 

numerous X-ray microprobe traverses. Thin and 

polished sections were prepared by M. E. Johnson and L. 
------------Aysees Member----------------.. Ranger Mountains Member 

above mound 

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FIGURE 2. - The carbonate mudmound on the west side of Meiklejohn Peak as seen from the north. Stratigraphic units indicated are the Lower Ordovician Ninemile Formation and the Paiute Ridge, Ranger Mountains, and Aysees Members (Byers and others, 1961) of the Antelope Valley Limestone. 

z 

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FIGURE 3. - Meiklejohn Peak and the mudmound as seen from relay station hill one-half mile to the west. Secret Pass lies in foreground (fig. 4) . Mudmound is about 300m 

(1,000 ft) wide and 80 m (270 ft) high. 

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FIGURE 4. -Approximate positions of lithologic samples and selected fossil collections in the carbonate mudmound (stippled). 

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01 
6 LITHOLOGY AND ORIGIN, MUD MOUND AT MEIKLEJOHN PEAK, NEVADA 

A. Wilson. R. E. Miller made a biochemical analysis of 

the laminated facies and established the presence of 

fatty acid and lipids. 

At the 1971 annual meeting of the Geological Society 

of America (Washington, D.C.) and at the 1972 annual 

meeting of the American Association of Petroleum 

Geologists (Denver, Colo.), we discussed most features of 

the mound with many geologists, among them M. J. 

Brady, John Harper, James Lee Wilson, Graham Davies, 

P. N. Playford, Perry Roehl, H. Zankl, Richard Rezak, 

Robert Dunham, P. W. Choquette, and R. W. Beales. 

Jaanusson and Ross were joined in an examination of 

the mound on August 2-3, 1972, by Nils Spjeldnaes, C. 

P. Hughes, R. A. Fortey, J. L. Henry, and L. R. M. 

Cocks. P. E. Cloud, Jr., joined Ross in a visit to the 

mound on April 14, 1973. 

GENERAL DESCRIPTION AND 

GEOLOGIC SETTING OF THE MUDMOUND 

As shown by Cornwall and Kleinhampl (1961) the 

geologic setting of the carbonate mudmound at Meiklejohn Peak is complex. The mound and surrounding 

strata are tilted tectonically with dips of 45° toward the 

east-northeast (fig. 2; Ross, 1972, fig. 4). The underlying 

calcarenitic limestone north of the mound has been displaced by a steep normal fault (Ross, 1972, fig. 3). The 

entire Ordovician section has overridden Devonian 

dolomite on a thrust fault. 

All three known Ordovician carbonate mounds in 

southern Nevada lie above limestone of the Paiute Ridge 

Member and within the Ranger Mountains Member 

(Byers and others, 1961), both constituting the lower 

member of the Antelope Valley Limestone (Ross, 1967, 

pl. 11). Immediately covering each mound is thinbedded, silty, nodular limestone. As shown by Ross 

(1972) the mounds lie within the Orthidiella zone; correlations by McKee, Norford, and Ross (1972) indicate 

that this zone is equivalent to the graptolite zones of 

Isograptus caduceus and Paraglossograptus etheridgei. 

The exact geometry of the carbonate mudmound is not 

known. In fact, we do not know what part of the mound 

is exposed to view, how much may be buried by covering 

sediments, or how much may have been eroded away. 

The exposed section suggests that the bottom is nearly 

flat, and that the mound is thickest in its middle and 

thinnest at the sides. It is about 300 m (1,000 ft) across 

its greatest visible dimension and 80 m (270 ft) high 

(figs. 3, 4). 

The laminated zebra limestone at the base of the 

mound is underlain by interbedded coarse calcarenite, 

dark-gray, silty, nodular limestone, and crudely 

laminated, partly stylolitic limestone; this underlying 

unit is about 9 m (30ft) thick and can be followed approximately 0.9 km (0.6 mi) northward above the 

Ninemile Formation. This unit is equivalent to the 

Paiute Ridge Member of the Antelope Valley Limestone 

on the Nevada Test Site. 

Although another mound is present at the same 

stratigraphic level east of the Nevada Test Site (Ross 

and Cornwall, 1961), its exposure is much smaller and it 

seems to lack the basal zebra limestone facies. We do not 

know whether that exposure represents a lateral tip of a 

large mound or the center of a small mound; the 

laminated facies may be present but hidden from view. 

The mound at Meiklejohn Peak shows evidence of 

several generations of fracturing, the details of which 

have not been explored. Clearly a system of cavities 

opened after formation of radiaxial fibrous calcite in the 

core and in the basal laminated beds. In many places 

cavities are filled with brown-weathering material. The 

cavities resemble distal parts of postdepositional fissures 

which are known in most other carbonate mounds. Such 

fissures are particularly conspicuous in the mounds of 

the Siljan district, Sweden, because they are filled by 

black graptolite shale. Within the core of the Meiklejohn 

Peak mound one can find several patches of intraformational conglomeratic material; it is also common in 

similar carbonate mounds elsewhere. 

The mound core is composed mainly of Stromatactisbearing calcilutite which shows only slight evidence of 

bedding. Some channels or pockets are filled with shells 

of cephalopods, brachiopods (particularly Idiostrophia), 

trilobites, and gastropods. In the upper part of the core 

Stromatactis constitutes about 20 per cent of the volume 

of the rock. 

The entire basal part of the core is laminated 

limestone composed of couplets of calcilutite (micrite) 

and radiaxial fibrous calcite. Each couplet is 6-10 mm 

thick. Following the example of Fischer (1964, p. 115), 

we call this basal facies zebra limestone. The aggregate 

thickness of the facies is highly variable, in some places 

being 20 em and in others 9 m. 

Covering the mound are thin beds of siliceously silty, 

highly fossiliferous, partly nodular, dark-gray limestone, 

whose silty parts weather grayish orange. Here and there 

in the lower third of the Ranger Mountains Member this 

lithology forms tongues into the mass of the mudmound. 

Two probable channels in the mound seem to have been 

filled by it. But the covering beds, particularly in the upper part of the mound, butt against the sloping sides of 

the mound and at the top cover it. 

Lithology of the part of the Ranger Mountains 

Member that forms the mound cover ( Orthidiella zone) 

was studied by Jaanusson in thin sections of a series of 

samples collected at the northern end of the mound. 

Jaanusson found that the covering limestone is 

preponderantly a sparitic calcarenite with the former 

voids between sand grains partially or completely filled 

with sparry calcite. Most of the limestone is a pelletal, 
LITHOLOGY 7 

sparitic calcarenite in which the majority of sand grains 

are cryptocrystalline pellets, mostly 0.08 to 0.2 mm in 

diameter. During deposition, the pellets were obviously 

indurated. The pelletal sand includes varying amounts 

of skeletal sand grains which dominate in some thin sections and form skeletal sparry calcarenite. The para- axial sparry calcite has to a large extent assimilated adjoining parts of skeletal grains making it difficult to determine the original grain boundaries. Therefore, it was impossible to apply modal analysis for quantitative determination of original sand constituents. 

In some places, the matrix is micritic, the interstices 

between sand grains being filled with carbonate mud, 

but such rock (micritic calcarenite) seems to be quantitatively less important than sparitic calcarenite. 

Radiaxial calcite was not observed in any of the thin sections studied. The irregular, argillaceous intercalations 

between limestone beds abound in quartz silt grains, 

mostly 0.02 to 0.06 mm in diameter. 

Jaanusson found that the limestone covering the 

mound is very different from that of the mound core. He 

concluded that a considerable winnowing of the sediment occurred and that deposition was in an environment with a much higher water energy than that during 

which the mound core was formed. 

LITHOLOGY 

Each lithologic facies of the mudmound was examined 

in thin section. Because its origin is highly controversial, 

the zebra limestone was subjected to other analyses. A 

semiquantitative spectrographic analysis was run by A. 

T. Myers. The laminae of calcilutite and radiaxial spar 

were analyzed for the isotopes of oxygen and carbon by 

Friedman. Insoluble residue was examined and identified by R. E. Wilcox and B. F. Leonard III. A sample 

was subjected to X- ray fluorescence and microprobe 

analysis by Ronald C. Surdam of the University of 

Wyoming. 

Representative thin sections of samples from the core 

of the mudmound were examined in detail by V. Jaanusson. 

LAMINATED LIMESTONE (ZEBRA LIMESTONE) 

By REUBEN JAMEs Ross, JR. 

GEOLOGIC RELATIONS 

At the north end of the carbonate mound, about 30 

feet (9 m) of the zebra limestone is strikingly exposed 

(fig. 5; Ross, 1972, fig. 6). Some individual laminar 

couplets may extend laterally for tens of feet (more than 

3 m) without break, but others are of much shorter extent. Lateral terminations of couplets are sedimentary 

rather than tectonic and account for variations in 

thickness of the laminated facies along the bottom of the 

mound. 

Although the laminated limestone forms the base of 

the mound core, it also occurs in restricted areas higher 

along the edges of the core. Wherever core mud has been 

deposited over channels or tongues of dark, silty 

limestone of the covering lithology the initial phase of 

mound deposition is laminated. An example was called 

to our attention by A. J. Rowell and F. F. Krause (Oct. 6, 

1971) about 80 feet (25 m) above the base and 200 feet 

(61.5 m) from the south end of the mound (USGS colin. 

D2334 CO; fig. 6). 

In one place near the middle of the base where the 

laminated rock totals about 35 em (1.2 ft) in thickness, 

two flat segments are connected without interruption by 

a quasi-flexure as if a very thick carpet were draped over 

a single step, the riser of which is also about 35 em (1.2 

ft) high. No fracture or tectonic displacement accounts 

for the flexure. 

Close to this place, about 3m (10ft) above the bottom 

of the mound, Professor Nils Spjeldnaes (Aug. 3, 1972) 

noted that the zebra limestone facies was interrupted by 

a cavity which itself was filled with shells of about 30 

nautiloid cephalopods and three generations of carbonate mud and sparry calcite. The visible, lens-shaped 

cross section of the cavity was almost 1 foot (30 em) in 

height and 2 feet (60 em) in width. Cephalopods were not 

confined to the youngest part of the cavity although the 

majority were "nested" therein. This disparity indicates 

that the laminated zebra facies had already formed early 

in the depositional and diagenetic regime because the 

cephalopods are the same as those found in the zebra 

limestone about 60 m (200 ft) farther south (R. H. 

Flower, written commun., 1972). 

CALCILUTITE LAYERS 

The laminated limestone exemplified by USGS colin. 

D2325 CO is composed mostly of alternating layers of 

calcilutite and of radiaxial fibrous calcite (fig. 7). Any 

attempt to categorize the kinds of calcilutite present or 

its relations with the radiaxial mosaic runs into complexities that defy simple interpretations. Any description of 

this rock seemingly involves speculation about its 

origins. 

In the simplest form, each layer of calcilutite consists 

of two parts (fig. 8): 

1. A lower fossiliferous calcilutite (cat), the upper and 

lower surfaces of which are highly irregular. Fossil 

material is contributed by ostracoda, trilobita, and 

echinodermata. Attitudes of fossils parallel layers 

of calcilutite (fig 9) except where ca1 is flexed or disrupted; there, fossils may rest at angles highly 

canted relative to the attitude of the laminae (fig. 

10). 

2. An upper pelletoid calcilutite (ca2) in which the 

diameter of the pelletlike particles is about 0.05 

mm. This material seems to fill depressions in the 

upper surface of the fossiliferous calcilutite. In 
8 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

FIGURE 5. - Cyclically laminated zebra limestone. This lithology is exposed through a thickness of 20 em to 9 m (1- 30ft) along 

the base of the mound. 

many cases all depressions are filled and a flat surface results (figs. 7, 8). 

In other places, a third, more homogeneous, unfossiliferous, equigranular calcilutite ( ca3) is present above 

the pelletoid mud (caz) and seems to hold the position of 

the radiaxial fibrous spar (fig. 11). This third calcilutite 

(ca3) locally appears crossbedded with pelletoid material 

(fig. 12) or intergrading with pelletoid calcilutite. Its 

usual position is above the pelletoid material (caz) and 

below the next higher fossiliferous calcilutite (cad (figs. 

11, 13). 

The zebra limestone is remarkable for its constant 
LITHOLOGY 9 

FIGURE 6. - Polished section of zebra limestone from south side of mound high on flank. All three calcilutites (ca ~, ca,, and cao) are developed. Disruption and deformation took place while ca2 and ca3 were saturated with water and hence semifluid . ca1 was more coherent, 

but not lithified. Radiaxial fibrous calcite (rax) formed only along bottom contact of ca1. Section illustrated in figure 40 cut along indicated line. USGS colin. 02334 CO. 

cyclicity whether at the bottom of the mound (figs. 7, 14) 

or above intertongued sediments higher on the sides of 

the mound (figs. 6, 11). I estimate that at the north end 

there are 900 cycles present within a thickness of 9 m. 

It should further be noted that when present the 

radiaxial fibrous calcite discussed below invariably occurs at the bottom contact of calcilutite ca 1 further 

emphasizing the cyclic nature of the rock. 

From the zebra limestone Jaanusson made a modal 

analysis of four separate layers of calcilutite ca 1 in thin 

section after visiting the mound in 1972. (For method see 

Jaanusson, 1972.) He found that the mean values of the 

main constituents are (1) matrix, 95 percent (range, 

92- 97 percent); (2) para-axial calcite, 2 percent (range, 

0.1-3 percent); and (3) skeletal sand, 3 percent (range, 

1- 6 percent). Compared with the bulk of the limestone in 

the massive mound core (mean values for matrix, 56 percent; para-axial calcite, 21 percent; radiaxial calcite, 16 

percent; and skeletal sand, 7 percent), the absence of 

radiaxial calcite and the low values of para-axial calcite 

in calcilutite ca1 of the zebra limestone are particularly 

noteworthy. Qualitative examination of peels and thin 

sections- too thick for a reliable modal analysis-of 

other ca1 layers suggests that these data are fairly 

representative. 

The average amount of skeletal sand is less in 
10 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

FIG!JHF: 7. Large section of laminated zebra limestone, showing 

cyclic nature. Calcilutite ca1 is fossiliferous; caz is pelletoid and fills 

hollows in top of ca1. Radiaxial fibrous calcite (rax) in every example 

is immediately below calcilutite ca1. In the bottom ofrax, a thin tier 

of clear radiaxial calcite (tr) is common. Compare with figure 8. 

USGS colin. D2325 CO. 

F IGURE 8. - Cyclic laminae. Depressions in upper surface of 

fossiliferous calcilutite (cad are filled or covered by pelletoid 

calcilutite (ca2). Space along median seam in radiaxial fibrous 

calcite (rax) opened during preparation of thin section. Lower tier 

(tr) of radiaxial calcite is well defined by micritic peloids along its 

top. USGS colin. D2325 CO. 

F IGURE 9. - Pelletoid material within radiaxial fibrous calcite (rax) 

is cut by median seam. Seam may be a secondary feature. Pelletoid 

material would not have been deposited unsupported in middle 

and upper half of cavity but could be relict calcilutite, partly converted by aggrading neomorphism. F, fossils. Thin section. USGS 

colin. D2325 CO. 
LITHOLOGY 11 

FIGURE 10. - Disrupted layers of zebra limestone contrast with those 

shown in figure 7, which are less than 60 em distant. Orientation of 

fossils in disrupted layers of calcilutite ca, might be interpreted to 

indicate that calcilutite ca , was divided by vertical crevices. USGS 

colin. D2325 CO. A, Thick section illustrating fluid appearance of 

calcilutite (ca,) between torn layers of ca,. Orientation of large fossil 

fragment (F) in center of section agrees with that of small fossils in B 

and C. B, Small part of section near top of A. Cross sections of fossils 

(F) are oriented at high angles to enclosing layers of calcilutite ca 1, 

therein contrasting with fossils in figure 9. Radiaxial calcite (rax and 

tr) might be lining of partly filled cavity, but would imply a different 

orientation than fossils. Radiaxial calcite may also have resulted 

from neomorphic conversion of preexisting aragonitic calcilutite. C, 

Small part of section near top of A and close to B. Note cross section 

of inarticulate brachiopod (ia) projecting downward into calcilutite 

( ca,) here made pelletoid. If boundaries of calcilutite ca, were fracture or shear surface, the fossil could not have survived. Despite attitude of seeming crossbeds of ca3, radiaxial calcite (rax) is 

developed preferentially immediately beneath each layer of ca,. 
12 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

FIGURE 11.- Thin section showing cyclic nature of laminae and 

selective aggrading neomorphism along contact between bottom of 

fossiliferous calcilulite (caJ) and top of equigranular calcitutite (ca,). 

Note t hat both ca1 and ca., are being converted to radiaxial calcite 

(rax) along this contact in both cycles. USGS colin. D2324 CO, from 

same sample as shown in figure 6. 

calcilutite ca1 than in the core but the ranges overlap. 

The difference is statistically significant at P=0.01level. 

This does not necessarily mean much, on account of the 

small area of ca1 layers analyzed. Nevertheless, it does 

indicate that the carbonate mud which formed ca1 layers 

was finer grained than most of the carbonate mud in the 

core. 

Owing to the small amount of skeletal sand present in 

the measured areas of ca1 layers, composition of the 

skeletal sand was calculated for all four analyzed areas 

together. The data, in percent, are as follows: 

Percent Echinodermata __ ____ _ ______ _ 52 

Trilobita _____ __ __ _ ________ 14 

Ostracoda __ __ ____ _________ 10 

Brachiopoda _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 3 

Mollusca____ ______________ 1 

Indeterminable _ _ _ _ _ _ _ _ _ _ _ _ _ _ 20 

Trilobites are more common than ostracodes, whereas 

m the core, the reverse is true; no "spiculae" were found. 

FIGURE 12. - In contorted or disrupted parts of zebra limestone 

cyclic succession of calcilutites may be disturbed. Here layers of 

calcilutite ca1 are separated by seemingly crossbedded interlayering of pelletoid and equigranular calcilutite(caa). Radiaxialfibrous 

calcite (rax) is immediately below ca1. In some cycles a tier (tr) of 

radiaxial calcite appears to underlie caa and is analogous to tr in 

figure 8. Polished surface. USGS colin. D2325 CO. 

Owing to the small area of ca1 layers measured, the low 

content of skeletal sand present, and the relatively high 

amount of small indeterminable grains, these data may 

not be representative for ca1 rock as a whole . But the 

analysis does suggest that the rock forming calcilutite 

ca1 was somewhat finer grained than most calcilutite in 

the massive core; its small content of drusy calcite indicates a diagenetic history different from that of the 

core. 

Jaanusson found that the pelletoid calcilutite (ca2) 

was too recrystallized (with many indistinct grain 

boundaries) to yield reliable data by modal analysis. 

RADIAXIAL FIBROUS CALCITE LAYERS 

The coarse calcite in the zebra limestone exhibits the 

essential characteristics of Bathurst's (1959, p. 511-512; 

1971, p. 426-427) radiaxial fibrous mosaic. In the 

simplest form, each radiaxial fibrous calcite layer is 

divided symmetrically into upper and lower halves by a 

median seam to which coarse crystals are roughly 

perpendicular. The seam is remarkably smooth despite 

the large side of the adjoining crystals (figs. 14, 15). Zoning in the coarse radiaxial calcite is disposed symmetrically above and below relative to the seam (fig. 

14A); the zoning is not necessarily parallel to the top or 

bottom contact of the spar. 
LITHOLOGY 13 

FIGURE 13. -Thin section cut normal to surface shown in figure 6 and 

parallel to surface shown in figure 40. Two cycles of fossiliferous 

calcilutite (ca d are shown. Pelletoid calcilutite (ca 2) and very fine 

equigranular calcilutite (ca3) complete the sedimentary cycle. Aggrading neomorphism has resulted in partial conversion of ca3 and 

bottom of overlying ca , to radiaxial calcite (rax). Calcilutite above 

ca :~ is not fragmental; if it were, it should also be spread to left on 

"floor" over ca 2• USGS colin. D2334 CO . 

In many places the radiaxial calcite layer is somewhat 

more complex. Below the thick symmetrical calcite layer 

is one, very rarely more than one, thin tier of radiaxial 

calcite. This thin tier can be distinguished in planetransmitted light with assistance of the pelletoid 

material incorporated in its bottom and in the bottom of 

the overlying radiaxial calcite (figs. 8, 14A, 14B). In 

polarized light, whether reflected or transmitted, the 

boundary between the upper thick symmetrical radiaxial calcite and the lower thin tier of radiaxial calcite is 

distinct only where peloids intervene; laterally the distinction between the two may disappear (fig. 14B). The 

tier is remarkably like the radiaxial calcite that occurs 

beneath seemingly crossbedded calcilutite in disrupted 

parts of the zebra limestone (figs. lOB, 16, 17). 

The crystals of calcite in the thick, symmetrical 

calcite tend to be largest close to the median seam and 

smallest along contacts with calcilutite (figs. 14C, 18). 

Large crystals exhibit undulate extinction and intergrown boundaries (figs. 14B, 14C). Relict extinction 

suggests that many large crystals have grown at the expense of their neighbors, be it by "cannibalism," 

aggrading neomorphism, or some other means. 

Pelletoid material and other bits of calcilutite are 

common within the radiaxial fibrous calcite. This 

material may be along the bottom of the thin lower 

radiaxial calcite tier or along the bottom of the lower half 

of the symmetrical thick radiaxial calcite (fig. 8). 

Although the pelletoid and calcilutitic material is 

usually below the median seam of the thick radiaxial 

calcite (fig. 15), it appears to be above or across the seam 

in some instances (fig. 9). 

At the south end of the outcrop and less than 3 m (10 

ft) above the base of the mudmound the zebra limestone 

overlies a tongue of dark-gray, nodular limestone of the 

covering facies (Ranger Mountains Member) and takes 

two forms. 

The first of the forms occurs immediately above the 

nodular limestone as irregular masses not more than 20 

em across and 10 em high composed of very thin discontinuous laminae of calcilutite ca, and radiaxial sparry 

calcite (fig. 19). There is a suggestion that these laminar 

bodies grew as small heads on the upper surface of the 

nodular limestone. Laminae range from 0.25 to 2.0 mm 

in thickness. 

The small finely laminated masses are reminiscent of 

a cross section through a head of cabbage. The thickness 

of radiaxial calcite between layers of calcilutite ca1 is 

about equal to the thickness of the layers of ca ,. 

Although space taken up by radiaxial calcite may once 

have been occupied by calcilutite, it seems equally likely 

here that radiaxial calcite has filled spaces between thin 

crusts composed of calcilutite ca1. 

Covering not only the nodular limestone but also the 

more finely laminated "cabbageheads" is the zebra 

limestone in a second, more conventional form. Here 

(fig. 20), it is composed of alternations of calcilutite ca1 

and pelletoid calcilutite ca2; in some couplets ca2 may 

have been deposited in two generations. Along the contact between ca2 and the next succeeding layer of ca1, 

radiaxial calcite has formed irregularly. 

Laterally, to the north, the layers of pelletoid 

calcilutite ca2 are taken over by radiaxial fibrous calcite. 

The field evidence suggests that the pelletoid calcilutite 

has been selectively replaced by or recrystallized into 

radiaxial calcite. Just as elsewhere along the base of the 

mudmound, the lateral extent of the zebra limestone 

facies, the thinness and irregularity of calcilutite ca, , 

and the even spacing of laminae argue against the 

mechanical possibility that the radiaxial calcite filled 

cavities. 

Chambers of nautiloids are filled with both para-axial 

and radiaxial calcite (fig. 21). Most contain para-axial 
14 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 
LITHOLOGY 15 

calcite; para-axial calcite occurs only as filling of 

nautiloid chambers or as filling of fractures that cut all 

other lithologies. Radiaxial fibrous calcite surrounds all 

shells except where they are still imbedded in calcilutite; 

one nautiloid chamber shown slightly left of center of 

figure 21 is mostly filled with radiaxial calcite but also 

contains calcilutite. Only the para-axial mosaic is certainly a cavity filling. The radiaxial calcite may have 

resulted from the conversion of calcilutite surrounding 

and in some places partially filling shells. 

TRACE-ELEMENT ANALYSIS 

A semiquantitative six-step spectrographic analysis 

was run by A. T. Myers on both the calcilutite and the 

radiaxial fibrous calcite in order to determine whether a 

significant chemical difference existed between them. 

The results (Myers, written commun., Dec. 1, 1971) were 

as follows: 

USGS colin. 0 232.5 CO 

Fe (percent) _ ______ _ _ Mg ______ _____ _ __ _ 

Ca ____ ______ ____ _ 

TI --------------- Si 

AI 

Mn (ppm) __________ _ 

Ba __ _ _ _____ ____ _ 

Cr _________ ____ _ 

Cu ___ ___________ _ 

Mo _ _______ ____ __ _ 

Pb ______ _____ ___ _ 

Sr ________ ____ _ _ _ 

v ____ ____ ______ _ _ 

Calcilutite 

.01 

0.3 

> 10 

0.01 

2.0 

0.15 

200 

70 

10 

15 

10 

20 

500 

20 

Radiaxial 

calcite 

.003 

0.3 

>10 

Detected 

.02 

0.01 

70 

50 

5 

5 

Not detected 

15 

500 

20 

FIGURE 14 (facing page). - Polished sections of zebra limestone. 

USGS colin. D2325 CO. A, Ordinary light; X 6. Two complete cycles 

of laminated sediment. Only part of zoning in radiaxial fibrous 

calcite (rax) is symmetrical about median seam. Lowest zonal band 

is lower tier of calcite (tr) as in figure 8. Calcilutites ca 1 and ca2 are 

difficult to differentiate under ordinary light. B, Polarized light; part 

of view shown in A; X 9. Within radiaxial fibrous calcite (rax) crystal 

faces are intergrown; shadowy relicts of small crystals are visible 

within larger crystals; the smaller, younger crystals tend to be at the 

top and bottom margins. Median seam (ms) is relatively smooth. 

Quartz, K-feldspar, and goethite stand in relief and are most abundant in fossiliferous calcilutite (ca,), less abundant in pelletoid 

calcilutite (ca2), and rare in radiaxial calcite. Lower tier of calcite 

(tr) is clear at middle but fades to right, where peloids have already 

been recrystallized to spar. C, Polarized light; small area of lower 

half of middle radiaxial fibrous calcite (rax) in B; X 40. Peloids (P) 

are relict, not having been recrystallized; therefore, the top of the 

lower calcite tier (tr) is distinct, and this tier may be comparable to 

tiers beneath ca3 in figure 168. Pelletoid calcilutite (ca2) under high 

magnification and polarized light appears crystalline intermediate 

between coarse radiaxial calcite and more finely crystalline ca 1• D, 

Polarized light; part of view shown in Band C. X 40. Quartz and Kfeldspar stand in relief; they are most abundant in calcilutite ca 1, 

less abundant in ca2, and very sparse in radiaxial calcite (tr and rax). 

FIGURE 15. - Thin section of three cycles in zebra limestone. Bright 

points within fossiliferous calcilutite (ca ,) are crystals of quartz and 

K-feldspar. Relict peloids and small bodies of calcilutite in radiaxial 

calcite (rax) may be comparable to those above calcilutite ca, in 

figure 13. USGS colin. D2325 CO. 

The calcilutite is consistently richer in trace elements 

than the radiaxial calcite, but the amounts concerned 

are not particularly unusual. The higher percentage of 

silicon in the calcilutite can be credited to the presence 

of quartz and feldspar. 

INSOLUBLE RESIDUE 

Photographs taken in polarized light of a polished surface of a sample from USGS collection D2325 CO (fig. 

14D) indicate the presence of material that is harder 

than calcite, particularly in the fossiliferous calcilutite 

(cat). Microscope examination of thin sections suggests 

that particles of quartz and an opaque mineral are the 

more resistant minerals seen in relief on the polished surface. 

Part of one sample from collection D2325 CO dissolved 

in dilute hydrochloric acid yielded 1.3 percent by weight 

insoluble residue. A second sample yielded 3.33 percent 

by weight insoluble residue. The particles of residue are 

almost entirely in the silt sizes; some particles are as 

large as very fine sand. By volume about 70 percent of 
16 LITHOLOGY AND ORIGIN, MUD MOUND AT MEIKLEJOHN PEAK, NEVADA 

FIGURE 16. (above and upper right) - Layers of fossiliferous calcilutite 

(cal) separated partly by radiaxial calcite (rax) and partly by 

seemingly crossbedded pelletoid calcilutite (caa), which also grades 

into radiaxial calcite. Pockets of calcilutite ca2 are overlain by a thin 

layer of radiaxial calcite (tr) just as in undisturbed parts of the zebra 

limestone. (See, for example, fig. 8.) Lower tier (tr) of radiaxial 

calcite may be analogous to that in figures 8, 13, 14A, and 14C. 

USGS colin. D2325 CO. A, Collodion peel. B, Polished surface. 

these particles are quartz, slightly fewer than 25 percent 

are potassic-feldspar, and about 7 percent are opaque. 

According to R. E. Wilcox (written commun., Feb. 25, 

1972), "Most of the particles of quartz and feldspar are 

made up of nuclei, probably detrital, about which have 

grown mantles of authigenic material to provide crude to 

well-developed euhedral crystal forms." According to B. 

F. Leonard III (written commun., Jan. 17, 1972), most of 

the opaque mineral is goethite. 

Assuming that the typical distribution of these insoluble minerals is shown on the polished section (figs. 14B, 

14D), we estimate that 70 percent are in the fossiliferous 

calcilutite (cat) and 25-30 percent are in the overlying 

pelletoid material (ca2). A trace (1-5 percent) is present 

in the radiaxial calcite, which makes up more than half 

the total rock. 

FIGURE 17.-Radiaxial fibrous calcite (rax) associated with seemingly 

pelletoid calcilutite between layers of calcilutite ca1 . Radiaxial 

calcite on right would not be discontinuous if it had lined a cavity 

prior to filling by calcilutite. Radiaxial calcite appears to have grown 

along more permeable sedimentary contacts. Pelletoid sediment 

may have provided permeable channel. Or pelletoid texture may be 

structure grumeleuse, resulting from aggrading neomorphism. As 

shown in figure 16, depressions above ca1 may be filled by pelletoid 

calcilutite (ca2); much of space between layers of ca1 is filled by 

typically equigranular ca3• Collodion peel. USGS colin. D2325 CO. 
LITHOLOGY 17 

In laminated sediments in Shark Bay, western 

Australia, quartz constitutes 1 percent of the sediment 

in the intertidal zone and 2-20 percent in the supratidal 

zone (Davies, 1970, table 1, p. 177). The small content of 

quartz in the laminated sediments at Meiklejohn Peak is 

therefore not unusual. 

Concerning Shark Bay, Davies (1970, p. 186) further 

noted that "pyrite or a related iron sulphide" is present 

in laminated sediments from the intertidal zone and that 

"gypsum is absent." Both these conditions are also met 

by the laminated facies from Meiklejohn Peak. After an 

X-ray microprobe analysis of a sample of this Ordovician zebra limestone, R. C. Surdam (oral commun., Apr. 

5, 1972) concluded that there is no difference in iron, 

calcium, or magnesium content between the calcilutite 

layers and the radiaxial fibrous calcite, and that the 

chemical suite present does not indicate deposition in a 

hypersaline or supratidal environment. 

rax 

FIGURE 18. (left and above) - Polished sections showing radiaxial 

fibrous calcite (rax) divided by nearly flat median seam (ms). Seam 

above calcilutite ca3 is jagged; terminations of crystals below seam in 

ca3 also seem jagged. Top of calcilutite ca3 partly converted to 

radiaxial calcite by aggrading neomorphism below seam. If conversion of ca3 continued, the tier of calcite (tr) below ca3 would correspond to lower tier (tr) in figures 8, 14B, and 14C. Polarized light. 

USGS colin. D2334 CO. A, "'X 20. A part of the sample shown in 

figure 41, this surface parallels and is 7 mm distant from that in the 

samples illustrated on figures 13 and 40. B, Enlarged view of part of 

A, X 39. 

A sample (USGS colin. D2343 CO) from the unlaminated upper part of the mudmound, 28 m (86 ft) 

above the base yielded a residue which is 2.16 percent by 

weight of the original sample. X-ray analysis by W. N. 

Sharp indicated that quartz and mica, probably illite, 

are the prominent constituents of this residue, and that 

some hematite is present. 

UNDERLYING LIMESTONE 

Northward from the mudmound the underlying 

calcarenite is exposed in a deep gully beyond which it is 

displaced westward by a fault. This calcarenite was initially considered to be a detrital apron derived from the 

mound; however, its continuity north and south beneath 

the zebra limestone of the mound shows it to be a part of 

the limestone 9 m (30ft) thick that lies stratigraphically 

below the mound and that extends over a wide area as 

the topmost part of the Paiute Ridge Member of the 

Antelope Valley Limestone. In the lower third of the 9-

metre-thick limestone a crudely laminated interval (figs. 

22, 23) resembles the zebra limestone but lacks the 

predominance of radiaxial calcite and the striking cyclic 

nature. 
18 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

FIGURE 19. -Thin section from small, irregular "cabbagehead" mass 

composed of calcilutite ca, and radiaxial fibrous calcite (rax). About 

10 feet (3 m) above base of mound at south end of outcrop. USGS 

colin. D2454a CO. 

In the calcarenite (fig. 24) recrystallization has 

obliterated the boundaries of skeletal grains making 

planimetric measurements of the original composition of 

the rock impossible. Some echinodermal material is present and ostracodes appear to be abundant. 

A collection (USGS colin. D2390 CO) was made at the 

bottom of the crudely laminated facies at a point where 

coarse sparry calcite is a minor constituent in the hope 

that the crude laminae might be analogous to the 

calcilutite layers of the overlying zebra limestone. 

A greater proportion of the rock is composed of fossiliferous calcilutite (figs. 22, 23). Layers, possibly of pelletoid origin (figs. 23B, 25), about 3 mm thick are interbedded and seem to be 'equivalents of the radiaxial 

calcite layers in the overlying zebra limestone. Spicules 

are present in clumps within the fossiliferous calcilutite 

(figs. 26, 23B). No such spicular clumps have been found 

in the zebra limestone. 

The poorly laminated limestone lacks the uniform 

cyclicity of the zebra limestone. The fossiliferous 

calcilutite ca1 forms rather irregular layers 5- 30 mm 

thick. Some of these layers have been bioturbated and 

locally the material has invaded lower layers in what appear to have been liquid protrusions from above; in other 

places there is as much evidence for upward flow of sediment as for downward flow. In still other places the sediment that appears to have been fluid in one spot has 

clearly been fractured a few millimetres away. Where 

fossils are abundant a considerable amount of pelletoid 

calcilutite is protected by "umbrellas" of larger shells. 

Within the fossiliferous calcilutite ca1 are numerous 

scattered dark-gray shapes in which spicules are prominent (figs. 23B, 26). Associated with the spicules is a fine 

anastomosing network of clear sparry calcite. After examining samples of the spicules and network, Robert 

Finks reported (oral commun., Nov. 9, 1972) that the 

straight spicules are probably root tufts of a lithistid 

sponge for which there is no other immediate evidence. 

He also thought that the anastomosing network of 

calcite was too irregular and devoid of overall geometry 

and spicular shape to belong to any sponge now known in 

the lower Paleozoic. In his opinion, this fine network 

might best be explained as the matrix in an original pelFIGURE 20. - Thin section from zebra limestone facies adjacent to 

"cabbagehead" mass shown in figure 19. Laminae composed of alternating calcilutite ca 1 and pelletoid calcilutite (ca2). Radiaxial calcite 

(rax) is incipient below ca, . Laterally layer ca2 becomes radiaxial 

fibrous calcite. At bottom of section note scour-and-fill(?) in ca2. 

USGS colin. D2454b CO. 
LITHOLOGY 19 

FIGURE 21. - A small part of a vertical section of a pocket or "nest" 

filled with shells of nautiloid cephalopods. All the space surrounding 

shells not now occupied by calcilutite (ca) is radiaxial fibrous calcite 

(rax). Almost all chambers are filled with blocky para-axial calcite 

mosaic (me). A few chambers are filled with radiaxial calcite (rax). 

One chamber slightly left of center is filled partly with calcilutite 

letoid calcilutite (Beales, 1958, pl. 1, figs. 1-3); Ross here 

considers this interpretation less likely for the network 

associated with spicules than for the somewhat similar 

and partly with radiaxial calcite. Presence of calcilutite may have 

been essential to formation of radiaxial fibrous calcite. Para-axial 

calcite (pa) occurs outside chambers only in fractures which cut all 

other lithologies. Collodion peel, photographed by transmitted light 

with one polarizer. USGS colin. D2389 CO. 

texture found in the thin (2-5 mm thick) layers that are 

interstratified with the fossiliferous calcilutite (figs. 23A, 

25A, 25B). Both textures are shown in figure 23B. 
20 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

FIGURE 22. - Polished section of poorly laminated limestone below 

mud mound. Dark-gray shapes contain spicular roots (sr) of sponges. 

Thin layers are pelletoid. Some thin layers and sparry calcite are fillings of internal cavities. USGS colin. 02390 CO. 

In texture the thin interlayer shown in figure 27 

somewhat resembles the late Paleozoic hydrozoan 

Palaeoaplysina described by Davies (1971, fig. 4 B, E). 

Dr. Davies kindly examined the sample illustrated and 

expressed the opinion (written commun., Nov. 3, 1972) 

that it was more readily interpreted as a pelletoid 

calcilutite with sparry matrix; however, he also suggested that the texture might be interpreted as a clastic 

biomicrite with algal(?) borings. 

Some of the thin layers in this crudely laminated 

facies contain sparry calcite and may have a different 

origin. These tend to be discontinuous and to cut across 

the fossiliferous calcilutite ca1. Many, but not all, appear 

to have been open fractures in the calcilutite, some cutting steeply across beds and some running nearly parallel. These presumed fractures are partly filled with 

geopetal sediment much of which was pelletoid. Most of 

the calcite bodies are entirely sparry para-axial mosaic; 

a few have radiaxial calcite enveloping the para-axial 

mosaic. The spar seems to have been formed in small 

discontinuous cavities. However, surrounding many of 

these calcite bodies is a halo of goethite at the contact 

with the calcilutite; such halos are common in replacement textures and it is difficult to explain the concentration of goethite within floor, walls, and ceiling of a 

cavity. 

MASSIVE CORE FACIES OF THE CARBONATE MUDMOUND 

By VALDAR JAANUSSON 

MACROSCOPIC CONSTITUENTS 

In the massive limestone forming the core of the 

mound, three main macroscopic constituents (here 

defined as structures larger than about 0.5 em) can be 

distinguished: (1) sparry calcite bodies (Stromatactis), 

(2) skeletal particles, and (3) the rest of the rock, 

macroscopically mostly of fine-grained appearance. 

STROMATACTIS 

The sparry calcite bodies are elongated in cross section 

and distributed throughout the core facies with the long 

axis approximately parallel to the former depositional 

surface. Their thickness rarely exceeds 2 em. The floor in 

most examples is fairly even, whereas the roof varies 

from even to digitate. Examination of these structures is 

made difficult by the sparsity of natural plane surfaces 

that are perpendicular to the depositional surface. Only 

a few such surfaces of sufficent size were found in the 

outcrop area of the mound core. Point counting in the 

field on three surfaces, each approximately 0.25 m2 and 

all located in the uppermost part of the mound, showed 

that the macroscopic sparry calcite bodies form about 

15, 17, and 18 percent of the volume of the rock, respectively. Examination of thin sections revealed that the 

sparry calcite bodies are composed of radial calcite 

(Bathurst, 1959) -that is, radiaxial fibrous calcite with 

undulose extinction, irregular, often highly digitate intercrystal boundaries, and numerous subgrains. 

The mode of occurrence, general shape, and 

microstructure of the sparry calcite bodies resemble 

those in what is generally known as Stromatactis, and 

the same term is used here for the structures from the 
LITHOLOGY 21 

core of the Meiklejohn Peak mound. The Stromatactis in 

the core differs from the sparry calcite layers in the zebra 

limestone by the much more irregular shape of the individual bodies, by their seemingly irregular distribution, and by their smaller size. The maximum observed 

length of a cross section of Stromatactis in the core of the 

mound is 24 em; most are considerably shorter. The 

microstructure of Stromatactis and the sparry calcite 

layers in the zebra limestone, on the other hand, is very 

similar. The radiaxial crystals or mosaic of crystals from 

FIGURE 23 (left and above) . - Polished and thin sections of sample of 

poorly laminated facies. USGS colin. D2390 CO. A, Polished section 

showing fossiliferous calcilutite (ca) separated by thin layers perforated by anastomosing tubes (tu). Tubes now filled with calcite 

mosaic, which may be matrix around pellets. Large dark-gray mass 

was produced by sponge and contains spicules (sp) of Pyritonema(?) 

and meshwork (M) of uncertain origin shown enlarged in Band C. B, 

Brachiopod(?) attached to long spicule (sp) paralleling plane of section appears as round spot in dark spicular mass inA . Discontinuous 

meshwork (M) below spicule may be sponge or calcite matrix around 

pellets of calcilutite. Abundant small white circles near top of 

photomicrograph are cross sections of spicules. Texture at bottom 

may be sparry calcite matrix around pellets or structure grumeleuse. 

Thin section. C, Thin section parallels B but does not cut brachiopod 

or large spicule. Large white circles are cross sections of 

Pyritonema(?) spicules. Peculiar discontinuous meshwork in center 

of view also appears below brachiopod in B. 

the floor and roof normally meet in a distinct seam (fig. 

28). 

None of the Stromatactis examined in the core of the 

mound exhibited a central, "residual" cavity or a welldefined central filling with para-axial (Bathurst, 1964) 

calcite. The absence of such structures may depend on 

the relatively small dimensions of Stromatactis in the 

Meiklejohn Peak mound or on their sparsity. 

The origin of Stromatactis has been, and still is, very 

much disputed and numerous different explanations 

have been proposed. Comparison of Stromatactis from 

the core of the Meiklejohn Peak mound with similar 
22 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

FIGURE 24. - Coarse calcarenite below mudmound, about 60 m (200ft) 

from the south end of outcrop. USGS colin. D2332 CO. 

structures from various other Paleozoic carbonate 

mounds suggests that the main factor in the origin is 

common to all. The material available for comparison 

includes Stromatactis from the Devonian of the Dinant 

Basin in Belgium (including samples from the quarries 

around Philippeville, the type area for Stromatactis Dupont, 1881), from the Ordovician of Sweden, and from 

the Carboniferous of England and Ireland. The material 

from the Meiklejohn Peak mound alone is too limited for 

forming a satisfactory basis of an extensive discussion of 

the whole problem of Stromatactis. Some observations 

presented here are deemed to be pertinent for understanding the origin of the structures. 

Several observations indicate that, irrespective of 

what the origin of Stromatactis may be, the shape 

and size of the radiaxial crystals, and possibly the whole 

radiaxial mosaic as such, is not original but a result of 

diagenetic processes. The radiaxial mosaic has 

demonstrably incorporated skeletal particles (Black, 

1952; Orme and Brown, 1963; Jaanusson, 1975) and 

grown at their expense. The material from the core of the 

Meiklejohn Peak mound shows numerous examples of 

this phenomenon (figs. 29, 30). A particularly interesting 

example (fig. 31) shows carapaces of ostracodes that occur in various stages of incorporation into radiaxial 

mosaic within a Stromatactis. Some of the carapaces are 

situated at the median seam of the radiaxial mosaic. The 

radiaxial mosaic has probably also assimilated finegrained matrix, although this cannot be proved. 

A mosaic which is very similar to the radiaxial mosaic 

can be shown to result from recrystallization (increase in 

the size of crystals) of finely fibrous normal calcite 

mosaic and, possibly, when aragonite is transformed into 

calcite in situ in closed space. Probably one of these 

processes was operative in the formation of radiaxial 

mosaic on Stromatactis. Subsequently, the mosaic grew 

by incorporating adjoining grains or extending into 

voids. The core of the Meiklejohn Peak mound contains 

numerous narrow veins filled with calcite. Some of the 

fissures probably formed early during the diagenetic 

history of the mound. The normal vein filling of these fissures is para-axial sparry calcite but where narrow veins 

cut a Stromatactis the calcite in the vein filling is 

generally radiaxial and in optical continuity with the 

crystals in adjoining radiaxial mosaic (fig. 15). This 

phenomenon has been observed in almost all carbonate 

mounds with Stromatactis from which material has been 

available. Orme and Brown (1963) suggested that this 

proves the mosaic of Stromatactis to postdate the formation of fissures, but in my opinion, a more plausible explanation is precipitation in a fissure with the crystal 

structure controlled by that of the wall. There are examples in the core of the Meiklejohn Peak mound where a 

vein cuts an echinoderm grain and the filling of the vein 

at that place is calcite in optical continuity with both 

halves of the echinoderm grain. 

Thus the present radiaxial mosaic of Stromatactis is 

most likely secondary. Determining the primary structure is difficult. This type of mosaic occurs in what 

demonstrably have been cavities in sediment (for example, in closed shells of brachiopods), but it is also known 

to replace original organic structures (stromatoporid 

coenosteums and stromatolitelike organic structures in 

the Upper Ordovician carbonate. mounds of Sweden). 

The presence of a distinct median seam might indicate 

filling of a cavity, because such seams have not been 

observed to be associated with recrystallization into a 

radiaxial mosaic. 

MACROSCOPIC SKELETAL PARTICLES 

Macroscopic skeletal particles are scarce in most of 

the core of the Meiklejohn Peak mound. The pointcounted surfaces mentioned previously (p. 20) did not 

include any skeletal particle of macroscopic size. 

Macrofossils tend in the core facies to be assembled into 

pockets or channellike structures but their importance 

relative to the total volume of the core of the mound is 

small. 
LITHOLOGY 23 

FIGURE 25. - Pelletoid(?) texture in thin light-gray layer similar to that below dark spicular mass in figure 23A and to that at bottom of figure 

23B. Thin sections of poorly laminated facies below mudmound. USGS colin. D2390 CO. A, Vertical section. B, Horizontal section. 

FINE-GRAINED LIMESTONE 

In some of the samples several generations of finegrained limestone can be distinguished; other samples 

are more homogeneous and contain possibly only one 

generation of micrite. The various generations differ 

mainly by microstructure and contact relations. Differences in color are so slight that recognizing the generations in the field is difficult, unless the rock surface is 

etched or stained, or both. 

The seemingly earliest generation of the fine-grained 

limestone (Li 1) is the commonest and resembles layer 

ca1 in the zebra limestone. It normally consists of cryptocrystalline micrite with varying amounts of skeletal 

grains (fig. 32). The skeletal grains of sand size are 

mostly more abundant than in ca1, and so the sediment 

is coarser grained than the limestone of the ca1 layers. 

What probably is the next generation (Li 2) has a more 

varied composition. Micrite similar to that of Li 1 is 

common, but the limestone also contains layers or 

patches of peloid limestone similar to that in ca2 of the 

zebra limestone recrystallized portions, and spar, 

radiaxial as well as para-axial (fig. 32). 

Where typically developed, the boundaries between Li 

1 and Li 2 are well defined (fig. 32). The complex distribution pattern of both generations suggests a system 

of burrows or borings in Li 1 filled with Li 2. With the 

material at hand, it is difficult to prove whether burrowing or boring was responsible for the pattern-whether 

the cavities had been formed in soft sediment or in a 

rock. Some observations, such as shells protruding from 

Li 1 into Li 2, suggest burrows, but the evidence is inconclusive. · 

The succession of the generations Li 1 and Li 2 is not 

always clear, and further studies may reveal a more complicated pattern of relations. 

In the available samples it is difficult to see any clear 

spatial relation between the two generations of finegrained limestone and individual bodies of Stromatactis, 

comparable to that between the layers of spar and ca 2 

layers of calcilutite in the zebra limestone or between 

Stromatactis and layers considered as internal sediment 

described from English and Irish Carboniferous carbonate mounds (Bathurst, 1959; Schwarzacher, 1961 ; 

Lees, 1964). In this respect further studies are necessary. 

The third generation of fine-grained limestone (Li 3) is 

relatively rare but distinctive . It mostly consists of a 
24 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

\ ~em 

FIGURE 26. - Polished section showing one of the dark masses 

illustrated in figure 22 (sr) where large spicules (sp) are particularly abundant and probably constituted roots of a sponge. 

USGS colin. D2390 CO. 

relatively homogeneous, fine-grained microspar with few 

skeletal grains. It generally occurs as a layer or fissurefilling at a steep angle or perpendicular to the former 

depositional surface. The boundaries against Li 1 and Li 

2 are sharp and accentuated by a thin layer of sparry, 

para-axial calcite, 0.07-0.15 mm thick. 

The relation of Li 3 to Stromatactis is illustrated in 

figure 33. There a small Stromatactis is faulted into four 

pieces, only three of which are visible in the 

photomicrograph (the fourth piece was just outside the 

right field of view). The almost straight margin of the 

Stromatactis to the right is formed by the fault which 

separates the third and fourth pieces. It cuts through a 

normally developed radiaxial mosaic in the same way as 

the first and second faults do. Faulting at the right 

formed a fissure, 2.6 to 2.9 mm wide, which is now filled 

with the limestone of the generation Li 3. The faulting 

also caused a vertical displacement of about 2.5 mm. 

This example shows conclusively that Li 3 postdates the 

formation of Stromatactis and that the general structural pattern of the radiaxial mosaic must have been 

fully developed before the deposition of Li 3. The sediment above and below the Stromatactis may still have 

been somewhat friable when faulting took place, as 

judged from the smooth curvature of the margin of the 

fault at some places (fig. 33). At other places, particularly where the wall is coated with spar, the wall of 

the fissure is straight. 

In parts of the mound the core is transected by 

macroscopically distinguishable, locally irregular fissures of varying width filled with brown-weathering 

limestone. This limestone obviously is still later than Li 

3. 

MICROSCOPIC CONSTITUENTS 

The microscopic composition of the limestone in the 

core of the Meiklejohn Peak mound was studied by 

modal analysis in a series of thin sections. The samples 

from which thin sections were prepared were intentionally taken from the macroscopically fine-grained 

parts of the core. Thus, the quantitative data apply only 

to this macroscopic component, comprising 80-85 perFIGURE 27. - Vertical section across thin layer of calcilutite perforated 

by irregular calcite-filled tubes, similar to those shown in figure 25. 

These may be sparry matrix around indistinct pellets or algal borings in calcilutite. USGS colin. D2390 CO. 
LITHOLOGY 25 

cent of the volume of the core. The general objectives of 

the study of microscopic carbonate constituents, as well 

as the methods, were reviewed and discussed by Jaanusson (1972). 

The series of samples for constituent analysis was collected by Ross in 1970 along a section through the middle 

part of the core of the mound (samples D2341 CO to 

D2349 CO; fig. 4) . In two thin sections (from D2341 CO 

and D2346 CO) recrystallization has nearly obliterated 

original grain boundaries; these thin sections were not 

measured. One thin section, from the sample D2349 CO, 

showed two clearly different rock types (calcilutite and 

calcarenite) separated by a sharp boundary; the areas of 

both types were measured separately. Patches of rock 

within almost all thin sections showed various degrees of 

aggrading neomorphism, and measurements involved 

more subjective judgments than is usual for such modal 

analysis. Still, qualitative study of peels from additional 

samples collected by Jaanusson in 1972 seems to indicate that the quantitative data are reasonably 

representative for the fine-grained limestone of the 

mound core. 

Owing to difficulties in distinguishing the generations 

Li I and Li 2 in most thin sections, these limestones are 

not discussed separately. The limestone of the generation Li 3 was not included in the measured area. 

FIGURE 28. - Thin section of a "microstromatactis" (ms) from the core 

of the mound. The surrounding limestone is either first (Lil )or second (Li 2) generation, except to the left, where a third generation of 

limestone (Li 3) is in contact with the radiaxial calcite. USGS colin. 

D2345 CO. Photograph by U. Samuelson of Naturhistoriska 

Riksmuseet. Lithologic sequence explained on p. 23. 

F IGURE 29. - Plate of an otherwise unidentified echinoderm partly 

converted to radiaxial fibrous calcite (rax). Thin section. USGS 

colin. D2345 CO. 

MAIN MICROSCOPIC CONSTITUENTS 

In the core of the Meiklejohn Peak carbonate mound, 

the main microscopic constituents (table 1) of the 

limestone are (1) sparry calcite, (2) skeletal sand, and (3) 

matrix . Sparry calcite was considered as a separate constituent only when a well-defined mosaic of calcite 

crystals was 0.1 mm long or larger. Skeletal sand is 

defined as those skeletal particles 0.1 mm long or larger 

in thin section (Jaanusson, 1952, 1972). The matrix -

the rest of the rock - consists of material of various 

origin, such as skeletal grains smaller than 0.1 mm, carbonate mud, terrigenous mud, and probably at least 40 

percent calcium carbonate cement. Much of the matrix 

is recrystallized into an ultramicroscopic mosaic of 

calcite crystals whose original constituents are no longer 

recognizable. 

Some thin sections contain dark, cryptocrystalline, 

pelletlike spots, almost completely surrounded by spar. 

In places they seem to represent indurated pellets, in 

others, it is difficult to prove whether they are 

depositional features (represent pelletlike sedimentary 
26 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

grains) or are phenomena developed during recrystallization (structure grumeleuse). Many of the spots are 

shorter than 0.1 mm and have indistinct boundaries. In 

the thin section D2343 CO, the peloids 0.1 mm long or 

larger comprise 8 percent of the measured area; in all 

other thin sections, they are much rarer or are absent. 

The peloids are tabulated as matrix. 

Modal analyses show that the fine-grained parts of the 

rock consist, in average, of 37 percent sparry calcite, 7 

percent skeletal grains, and 56 percent matrix. 

SPARRY CALCITE 

About 44 percent of the microscopic sparry calcite is 

radiaxial and the rest is para-axial. The microscopic 

bodies of radiaxial sparry calcite (fig. 28; Ross, 1972, fig. 

7) resemble Stromatactis in microstructure and general 

shape. For convenience they are termed here 

FIGURE 30. - Thin section from USGS colin. D2342 CO showing 

fossil ostracodes (os) and pieces of calcilutite (ca) relict in radiaxial 

fibrous calcite (rax) . 

- Thin section of a "microstromatactis" with incorporated ostracode carapaces (arrows) in various degrees of 

recrystallization into radiaxial calcite. USGS colin. D2344 CO. 

Photograph by U. Samuelson, Naturhistoriska Riksmuseet. 

FIGURE 32. - Thin section from the core of the mound showing 

distribution of two generations of limestone (Li 1 and Li 2). The 

large sparry calcite mosaic in Li 2 is radiaxial, as is the spar inside 

a partially closed brachiopod shell (br) which is mostly filled with 

micrite. USGS colin. D2343 CO. Photograph by U. Samuelson, 

Naturhistoriska Riksmuseet. 
LITHOLOGY 

"microstromatactis." There are all gradations in size 

from microstromatactis (with a thickness less than 1 

mm) to Stromatactis of macroscopic dimensions. Other 

microscopic mosaics of radiaxial calcite have less regular 

outlines and lack a clear median seam. The nature of 

such mosaics is not always clear. Some represent 

peripheral cuts of microstromatactis; others suggest formation by aggrading neomorphism. 

Where former original cavities can be demonstrated, 

such as within closed carapaces of ostracodes and closed 

shells of brachiopods, the cement is mostly radiaxial 

calcite. Some such intragranular cavities contain a 

central filling of para-axial calcite which is later than the 

radiaxial calcite. 

FIGURE 33 (right). - Thin section of a "microstromatactis" which is 

faulted into four pieces (the fourth piece is to the right just outside 

the lower right corner of the photograph). The fissure at the right is 

filled with limestone of the generation Li 3. USGS colin. 02343 CO. 

Photograph by U. Samuelson, Naturhistoriska Riksmuseet. 

TABLE 1. - Relative proportions of microscopic constituents for six collections taken 

stratigraphically through center of the carbonate mudmound at Meiklejohn Peak 

Meters 

80 

70 

60-

50 

20 

[Positions of collections indicated in fig. 4. Collection 02349 CO from nodular limestone immediately covering moundl 

USGS 

Ft. Colin. No. 

02349 

02348 

02347 

200 

02345 

02344 

02343 

02342 

0 Base of mound 

Matrix 

0 20 40 60 80 

Sparry calcite 

- Para-axial 

CJ Radiaxial 

= =-~ 

0 20 40 60 

PERCENT 

Skeletal 

sand 

~ 

~ 

• 

~ 

~ 

~ 

0 20 40 

27 
28 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

TABLE 2. - Relative proportions of organic components of the skeletal sand fraction shown in table 1, for six collections taken 

stratigraphically through center of the carbonate mound 

[Positions of collections indicated in figure 4. Collection D2349 CO from nodular limestone immediately covering mound] 

USGS Spiculae-like Trilobita Bryo zoa 

Meters Ft. Colin. No Echinodermata rods Ostracoda Brachiopoda Mollusca Nuia Algae Indeterminate 

0 2349 

80 0 2348 ~-~ ::"" 

70 

02347 ~ !- - 60 200 

02345 

-

02344 ~~ ~ 

02343 I-ll ~ 

20 

02342 ~ ~ 

0 20 40 60 0 20 40 0 20 

Para-axial calcite frequently forms relatively coarse 

mosaics similar in size and shape to microstromatactis 

except that the outline tends to be much more irregular 

and a median seam is seldom present. Such para-axial 

mosaics may belong to a generation different from that 

of radiaxial calcite but this could not be proved. Where 

both types of sparry calcite mosaics are in contact, the 

boundary between the mosaics is generally transitional. 

However, this may be a phenomenon of the same nature 

as the development of radiaxial calcite in those parts of 

narrow veins that cut a radiaxial mosaic; the structure of 

the crystals acting as seeds may have become 

propagated beyond the original boundary of a mosaic. 

Radiaxial and para-axial sparry calcites tend frequently to be concentrated in different patches- in different samples or in different parts of a thin section. This 

affects, then, not only the relatively large mosaics but 

also intragranular and intergranular cement. Near the 

top of the mound para-axial sparry calcite 

predominates, whereas in the lower and middle parts of 

the mound radiaxial calcite predominates. The implication of these differences in distribution is not clear. 

!I' r - =- = • • 

~ !- .. 

-· 

~ ... 

~ ~ ~ ... 

II 1- ~ .. "' 

~ !- .. 

0 10 0 10 0 10 0 10 0 20 0 10 0 20 

PERCENT 

Skeletons which were originally aragonitic (here 

regarded as part of the skeletal sand) have mostly been 

replaced by a para-axial sparry calcite even in areas 

where radiaxial calcite dominates. This might suggest 

that the transformation of aragonite to calcite was later 

than the formation of intragranular radiaxial spar. 

However, there are exceptions and further studies are 

needed. 

Much of the microscopic para-axial sparry calcite 

forms irregular patches of relatively fine-grained mosaic 

which obviously formed by aggrading neomorphism, as 

defined by Bathurst (1971, p. 481). 

COMPOSITION OF SKELETAL SAND 

In the thin sections studied the amount of skeletal 

sand varies between 4 and 11 percent (table 1), and thus 

the rock can be characterized as a calcilutite. 

Calcarenitic areas do occur, particularly in pockets 

abounding in skeletal remains, but they are quantitatively unimportant. The sediment was mudsupported. 

Within the skeletal sand (table 2) echinoderms 
LITHOLOGY 29 

FIGURE 34. - Spiculaelike skeletal grains showing rays composed of para-axial sparry calcite. USGS colin. 02343 CO. Photographs by 

U. Samuelson, Naturhistoriska Riksmuseet. A, Three of four rays shown; X 35. B, Four rays; the ray to the left merges into radiaxial 

sparry calcite; X 100. 

predominate by volume . Second in volume are straight 

spiculaelike rods. Some show four short rays (fig. 34B) 

and thereby resemble pentact megascleres of sponges, 

whereas others have four relatively long rays (fig. 34A). 

In thin sections the "spiculae" mostly occur as circular 

to elliptical cross sections of rods. Some may be 

Pyritonema, which is similar in microstructure and 

general dimensions. The original composition of the 

skeleton was probably aragonite because the "spiculae" 

are now a relatively coarse para-axial mosaic of sparry 

calcite . This often hampers recognition of the 

spiculaelike skeletal grains; some spiculae either have 

not been distinguished from sparry calcite or have been 

confused with other groups which originally had 

aragonitic skeleton, such as mollusks or an enigmatic 

tubelike microfossil with a very thin wall. Many tubes 

are filled with sparry calcite and then the wall is difficult 

to recognize. Aragonite as a skeletal mineral is, to the 

best of our knowledge, not reported among sponges and 

the attribution of the spiculaelike skeletal grains is at 

present uncertain. 

By volume ostracodes predominate over trilobites. In 

this respect composition of skeletal constituents in the 

core of the Meiklejohn Peak mound differs from that of 

many other Ordovician limestones. 

A component in the skeletal sand of the Meiklejohn 

Peak mound, not known from European Ordovician 

limestones, is the enigmatic Nuia, widespread in Lower 

and Middle Ordovician limestones of North America 

(Toomey and Klement, 1966; Johnson, 1966; Toomey, 

1967). Originally the skeleton of this possible alga 

probably was calcite. 

COMPARISON WITH SELECTED OTHER 

PALEOZOIC CARBONATE MOUNDS 

In several respects the Meiklejohn Peak mound resembles certain other Paleozoic carbonate mounds, such as 

the Ordovician Kullsberg and Boda Limestones of the 

Siljan district of Sweden and the Carboniferous "reef 

knolls" of the British Isles and Belgium. In all these 

mounds, as well as in several others, Stromatactis is an 

important macroscopic constituent. In the Kullsberg 

Limestone Stromatactis forms about half the volume of 

the rock and in northwestern European Carboniferous 
30 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

mounds the importance of Stromatactis seems to be 

closely comparable. The Meiklejohn Peak mound differs 

in its smaller content (15-20 percent) of the sparry 

calcite bodies. 

In all these carbonate mounds microscopic sparry 

calcite is abundant, radiaxial as well as para-axial. In 

the Kullsberg Limestone it forms, in average, about a 

third of the volume of what macroscopically is finegrained limestone (Jaanusson, 1975). Microscopic sparry 

calcite is abundant also in thin sections examined from 

the northwestern European Carboniferous mounds. In 

the Meiklejohn Peak mound the macroscopically finegrained limestone of the core abounds in sparry calcite, 

averaging about a third of the volume of the rock. A part 

of the sparry calcite may have been precipitated as cement but a part is demonstrably formed by aggrading 

neomorphism, that is, by recrystallization of sedimentary particles and carbonate cement into sparry calcite 

mosaic after aragonite had either dissolved or been 

transformed into calcite. The carbonate mounds in 

Sweden, Belgium, and the British Isles consist of 

limestones of high purity (97-99 percent CaC03 ; in the 

core of the Meiklejohn Peak mound the amount of insoluble residue is about 2-3 percent). A contributing factor to the extensive recrystallization of such limestones 

may be the low content of terrigenous material, particularly clay minerals (Zankl, 1969) and other impurities which can hinder crystal growth. 

The skeletal constituents of the Meiklejohn Peak 

mound (table 2) differ markedly from those of the Ordovician mounds of the Siljan district, Sweden; (Jaanusson, 1975), the Devonian Upper Koneprusy mounds in 

Bohemia (Jaanusson, 1975), and the Carboniferous "reef 

knolls" of the British Isles and Belgium. In all those carbonate mounds fenestrate bryozoans (phylloporinids in 

the Ordovician mounds and fenestellids in the Devonian 

and Carboniferous mounds) are abundant and commonly are the predominant component of the skeletal 

sand. Abundance of fenestrate bryozoans has been 

reported also from several other carbonate mounds 

(Pray, 1958; Cotter, 1965; and others). The abundant 

fronds of fenestrate bryozoans formed grain-supported 

sediment; not all the extensive intergranular voids 

became filled with sediment; some voids persisted and 

sparry calcite cement was subsequently precipitated 

therein . The fenestrate bryozoans may also have acted as 

sediment traps and contributed to the growth of the carbonate mounds. 

In the Meiklejohn Peak mound bryozoans are very rare 

and no fenestrate form has been found. Bryozoans are 

much more common in the upper member of the 

Antelope Valley Limestone well above the mound but do 

not form an important constituent of the skeletal sand 

therein. 

Many Paleozoic carbonate mounds show evidence of 

an early lithification. That a mound was lithified prior to 

deposition of the covering beds can be demonstrated in 

numerous examples. During final settling of a mound, 

broad dilatational crevices were formed in the thenlithified rock and subsequently filled with sediment from 

above. (See Isberg, 1917, 1918, and others for the Ordovician carbonate mounds of the Siljan district, Sweden; 

Chlupac, 1955, for Devonian Upper Koneprusy "reefs" of 

Bohemia; Philcox, 1963, for Carboniferous "reef knolls" 

of Ireland; Pray, 1965, for "banks" of Waulsortion type 

in the Sacramento Mountains, New Mexico; Cotter, 

1965, for Mississippian "banks" of central Montana.) In 

the Ordovician carbonate mounds of the Siljan district, 

Sweden, it can be shown that not only was the mound 

lithified before formation of the crevices but the main 

features of the radiaxial mosaic in Stromatactis were 

already fully developed (Jaanusson, 1975). This proves 

that Stromatactis formed before development of the 

crevices. This relation sets an upper limit for the time of 

formation of the radiaxial mosaic in Stromatactis and 

the lithification of the mound. 

The Meiklejohn Peak mound apparently had no major 

postdepositional crevices comparable to those in other 

mounds. However, fissures are known which were obviously formed in a lithified sediment before the growth 

of the mound had ceased. The main features of the 

radiaxial mosaic of Stromatactis can be shown to have 

been developed when the mound was still growing. This 

agrees with the suggestions by Lecompte (1936, 1937), 

Schwarzacher (1961), and Lees (1964) as to the very 

early formation of the spar in Stromatactis. 

ISOTOPIC INTERPRETATIONS 

By IRVING FRIEDMAN 

The contrasting components of the zebra limestone, 

the beds covering the mudmound, and cement from 

breccias and fractures that cut both the mound and 

covering beds were analyzed for the isotopes C13 and 0 18 

(tables 3, 4). 

All isotopic data are given in per mil. The oC13 values 

are in respect to PDB (Peedee belemnite), and the o0 18 

is given in respect to SMOW (standard mean ocean 

water). The isotopic data are given in per mil and are 

precise to ±0.1 percent. 

The oC13 of the samples is plotted on a frequency composition diagram, figure 35. The oC13 compositions of the 

calcilutite and coarse radiaxial and para-axial calcite are 

very similar and equal to the oC13 of modern marine carbonate. The small spread in oC13 values indicates that 

they formed from a well-mixed carbon reservoir and that 

carbon from organic matter was an unimportant component. Organic carbon has a oC13 of - 8 to - 26 per mil, 

compared to present-day ocean water bicarbonate of - 2 

per mil. The covering beds, by contrast, show great 

variability in oC13 , and about half the samples have a 
ISOTOPIC INTERPRETATIONS 31 

oC1:J that is much lighter than any of the calcilutite or 

coarse calcite samples. This enrichment in C12 and the 

variability of oC13 can be accounted for if the covering 

beds formed in an environment in which C02 from 

organic matter formed an important but varying part of 

the C02 reservoir, in addition to the normal atmospherederived ocean water bicarbonate. 

Keith and Parker (1965) found that the oC13 of mollusk 

shells growing in marginal marine environments was 

dependent upon the accessibility of continental (land 

plant-derived) carbon, and that the oC13 values of the 

samples approached the marine values as the sampling 

sites progressed from estuarine to marginal bays. The 

samples from marginal bays also show a variable oC13 , 

which Keith and Parker (1965, p. 127) attribute to 

"variable effects due to locally-produced COz from 

decompositon of organic detritus, both continental and 

marine (Landegren, 1954) and from respiration of 

aquatic plants. Development of a local C13 deficiency by 

aquatic plants in waters with restricted circulation is 

consistent with observations by Wickman (1952)." 

That the covering beds contained organic matter is 

shown by the rich fauna (trilobites and particularly 

brachiopods). The decomposition of the soft parts 

probably contributed light COz to the pore water. 

Restricted circulation resulted in some of this C02 being 

incorporated into the shells of organisms living in this 

environment. 

In the mudmound the similarity of oC13 between the 

calcilutite and the coarse calcite indicates either that 

both formed from the same well-mixed, large carbon 

reservoir, or that one formed from this reservoir and the 

other formed by recrystallization of the first-formed 

material without a significant admixture of carbon from 

decomposing organic matter. 

The cause of this recrystallization can be found in the 

greater solubility of the fine particles as compared to the 

coarse spar crystals. The large surface area of the 

calcilutite particles results in greater surface energy and 

therefore less stability for those grains than for the large 

calcite crystals. The process of solution of the finest 

grains and growth of the coarse calcite would have been 

aided by small temperature oscillations, which would 

have occurred before deep burial of the mound. The 

process probably almost stopped when burial was deep 

enough to damp out diurnal and seasonal temperature 

fluctuations, and this may account for the persistence of 

some calcilutite. 

Possibly, the recrystallization was facilitated because 

some of the original calcilutite was not calcite but a form 

of calcium carbonate (aragonite or vaterite) which is 

thermodynamically less stable than calcite. 

The isotopic data for the cement of the dolomite fault 

breccia show that this material is similar in o018 values 

to the calcilutite and spar, and probably formed in Ordovician time. If it formed from fresh water, a lower o018 

value would be expected, and if it formed from seawater 

in later times, its o018 value should be heavier. 

Figure 36 is a frequency o018 diagram. The samples 

from covering beds and calcilutite have similar ranges of 

composition, from +20.3 to +22.5. The coarse calcite 

covers a slightly larger range, from +20.3 to +23.2, with 

a clustering of values between +21.8 and +23.2. The 

radiaxial fibrous calcite therefore has about the same 

o018 value as do covering beds and calcilutite, but appears to be slightly enriched by about 0.5 per mil in 0 18 . 

One explanation for the rather small spread of o018 

values is that the carbonate formed under marine conditions with little influence from fresh water. Another explanation is that all the samples have been altered and 

their o018 content modified. However, if alteration had 

occurred with water of different o018 than the original 

seawater, we would expect either that the fine submicron 

calcilutite would have exchanged more than the very 

coarse millimetre-sized radiaxial calcite grains, or that 

they both would have exchanged completely. This latter 

case would have resulted in a very uniform o018 content 

rather than the range of o018 values found. The first case 

would have resulted in the calcilutite having much 

lighter o018 values than the coarse calcite. The similarity 

TABLE 3. - Isotopic analysis of beds of Antelope Valley Limestone 

around mudmound, shown in fig. 4 

Sample 

No. 

[All values are given in per mil) 

Description 

3376-30--- Middle, thick-bedded Aysees 

Member about 50 ft above 

highest point of mudmound. 

D2335 CO. Dolomitized _- 29 __ _ 

47---

48 __ _ 13 __ _ 

49 __ _ 

50 __ _ 

Dolomite fracture filling in 

D2335 co ----------

Covering beds of Ranger Mountains Member abutting 

bioherm, approx. 200 ft above 

base of mound. D2340 CO. 

Calcilutite fraction ____ _ _______ do __ - ----

Covering beds of Ranger Mountains Member abutting 

bioherm, about 195 ft above 

base of mound. D1970 CO 

Covering beds of Ranger Mountains Member, about 5 ft 

below D1994 CO, about 108 ft 

above base of mound D2326 

CO. Dark-gray fine-grained limestone _________ -

Covering beds of Ranger Mountains Member, about 5 ft 

below D1994 CO, about 108ft 

above base of mound 02326 

CO. Grayish-orangeweathering calcarenite __ _ 

32 _ _ _ Covering beds, close to mound on south side, about 150 ft 

above base. Worm tubes. 40ft 

-1.0 

-1.6 

-2.5 

-3.8 

-1.6 

-1.4 

-1.2 

oo1s 

+21.4 (20.6) 

+19.2 

+20.6 

+20.6 

+21.8 

+21.5 

+21.5 

south of D2334 CO. (71/30) -0.5 +21.7 
32 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

TABLE 4. - Isotopic analysis of limestone from main core of mud mound and from zebra limestone 

["Spar" refers to radiaxial fibrous calcite in this table. Samples are listed stratigraphically from top to bottom. All values are given in per mil J 

Sample No. Description 

3376-52 _____ Nodular limestone capping mound. 

USGS colln. 02338 CO. ____ - 52 _ _ _ _ _ Para-axial calcite in "birdseye" D2338 

co. ----------------- 53 _ _ _ _ _ Topmost outcrop of mudmound (71/2). 

02337 co. ------------- 51 _____ Topmost outcropp of mudmound 

(71/2). D2348 CO. --------- 32 _ _ _ _ _ Cephalopod, chamber filling, 170 ft 

above base of mound (71/27). 02331 

CO. Inner part----------- 32 _ _ _ _ _ Cephalopod, chamber filling 170 ft 

above base of mound (71/27). 02331 

CO. Outer part----------- 35 _____ Lens of covering bed lithology, 30 ft 

south of 02331 CO. D2333 CO. _ 27 _ _ _ _ _ Laminated limestone, 143 ft above 

base of mound. 02344 CO. ___ _ 28 _ _ _ _ _ Random sample, zebra limestone, less 

than 40 ft above base of mound. 

Sp~ ---------------- Calcilutite, fossiliferous ____ _ 

Sp~ ---------------- Calcilutite, fossiliferous ____ _ 

---------~ -------- Calcite vein, crosscutting ___ _ 

43 _ _ _ _ _ Calcilutite, fossiliferous 1 

42----- Spar, in upperd~k zone1 ______ _ 

36 _____ Spar, below median seam1 ______ _ 

37 _____ Spar, lower dark zone 1 _______ _ 

38 ___ - _ Calcilutite, fossiliferous1 ______ _ 

44 _____ Spar1 ------------------

45 _ _ _ _ _ Calcilutite, fossiliferous1 ______ _ 

46 _____ Spar1 ------------------

41 _____ Calcilutite, fossiliferous1 ______ _ 

40 _____ Spar, two dark zones above seam1 __ 

39 _____ Spar, above median seam1 _____ _ 

18 --_-- Spar and calcilutite lamina2 ____ _ 

24 _____ Spar, upperhalf2 ------------ 23----- Spar, lowerhalf2 ------------ 22- ____ Calcilutite, fossiliferrous 2 ______ _ 

21 _____ Spar, upperhalf2 ------------ 20 _____ Spar,lowerhalf2 ------------ 19 _ _ _ _ _ Calcilutite, fossiliferous 2 ______ _ 

3379- 8 _____ Calcilutite, fine, light gray, wackestone3 ____________ _ 

9 _____ Spar, filling gastropod shelP ___ _ 10 _____ Spar, lining channel (?)3 _____ _ 

7 _ _ _ _ _ Calcilutite, fossiliferous adjacent to 

spar above seam4 ---------

6 _____ Spar, outer zone, above seam4 

5 _____ Spar, at center, above seam4 ___ _ 

1 _____ Spar, at center, below seam4 ___ _ 

2 _____ Spar, outer zone, below seam4 __ _ 

3 _ _ _ _ _ Calcilutite, pelletoid4 _______ _ 

4 _ _ _ _ _ Calcilutite, fossiliferous4 ______ _ 

3376-54 _ _ _ _ _ Calcarenite, 15 ft below base of mound near center. D2330 CO. _____ - 3376-55 _ _ _ _ _ Para-axial calcite in fracture or seam cutting calcarenite _______ - - 3379-19 _ _ _ _ _ Black calcilutite, fine 1 _______ _ 

20 _ _ _ _ _ Black calcilutite, coarse1 _____ _ 

21 _____ Brown calcilutite1 _________ _ 

22 _ _ _ _ _ Black pelletoid calcilutite1 

23----- Spar1 ------------------ 24 _____ Brown calcilutite1 _________ _ 

25 _ _ _ _ _ Spar1 _________________ _ 

Sparry calcite 

-0.9 

-0.5 

+0.3 

0.0 

-0.2 

-0.1 

-1.8 

-0.4 

-0.1 

-0.3 

0.0 

-0.2 

-0.3 

-0.1 

-0.1 

-0.1 

0.0 

+0.3 

+0.1 

-0.1 

+0.2 

+0.1 

0.0 

+0.2 

+0.1 

+0.2 

-0.1 

+0.3 

-0.7 

-1.1 

+0.2 

li0'8 

+22.5 

+21.4 

+21.5 

+21.7 

+21.5 

+20.5 

+13.6 

+21.9 

+21.8 

+21.8 

+22.4 

+21.4 

+22.0 

-22.4 

+22.4 

+22.4 

+22.7 

+22.6 

+22.4 

+22.4 

+22.4 

+22.1 

+22.2 

+23.2 

+22.4 

+20.3 

+22.0 

+22.2 

+20.4 

+21.0 

+22.8 

Calcilutite 

-1.1 

-0.5 

-0.3 

+0.1 

+0.4 

-0.2 

-0.2 

-0.2 

-0.6 

-0.5 

-0.1 

-0.5 

-1.0 

-1.0 

-0.4 

0.0 

-0.2 

-0.4 

+0.2 

+0.4 

-0.1 

-0.1 

+0.3 

-0.7 

+0.4 

-0.1 

-0.2 

0.0 

li0'8 

+21.8 

+22.1 

+19.6 

+22.3 

+21.6 

+21.3 

+20.4 

+20.6 

+21.3 

+21.0 

+20.9 

+20.9 

+20.4 

+20.4 

+20.3 

+20.7 

+20.7 

+20.9 

+22.3 

+21.3 

+21.1 

+21.3 

+22.2 

+20.5 

+20.6 

+20.5 

+22.1 

+20.6 

1 Zebra limestone, approx. 60ft above base of mound. D2325 CO. This sample taken from a huge block, which lies askew to surrounding rock. A. J. Rowell (oral commun., 1971) proposes that this block may have tumbled from a part of the mound long 

since removed by erosion, perhaps in Ordovician time. This possibility needs consideration. 

•Random samples, zebra limestone above base of mound at south end. 

"Channel fill, near base of mound. D2324 CO. 

'Below basal bed of mudmound, poorly laminated limestone (no calcarenite present). D2327 CO. 
ORIGIN OF THE MUDMOUND 33 

2~--------------------------------------~-------------------------------------------, 

en 

w 

4 

3 

2 

~ 1 

~ 

~0 

u.. 

0 

a:7 

w 

c:::J 

~6 

::> 

z 5 

4 

3 

2 

0 

rrr1--

-

-

-

-

-

-4 

Dolomitized 

Calcilutite 

I I 

I I 

-3 -2 

Oolomitized 

Covering beds 

-

-

I 11 l I I I 

-

-

-

Radiaxial calcite -

-

-

I J II -

I 

-1 0 +1 

FIGURE 35. -Frequency composition diagram for oC1a in samples of calcilutite and radiaxial calcite from zebra limestone, from 

mound core, and from limestone of covering beds. 

in o018 values between the two precludes any important 

amount of recrystallization in fresh {light o018) water. 

The isotopic evidence from oC13 and o018 is consistent 

with the spar's having formed by recrystallization of the 

fine-grained calcilutite. Further, this recrystallization 

must have taken place in water of the same isotopic composition as that in which the calcilutite originally formed 

and at a temperature, in general, the same as or 5°-10°C 

cooler than the temperature of formation of the fine lime 

mud. 

From evidence already presented we have concluded 

that later alteration of the calcite by fresh water was unimportant, and that the calcite (or aragonite) has the 

same o018 content as it originally had. Shell material 

forming in the present marine environment has a o018 

value of +30 to +32 (Epstein and others, 1951). Forming 

carbonate of +21.5 per mil from water of 0 per mil 

(present-day ocean) requires a temperature of about 

65°C. An alternate explanation for the +21.5 per mil is 

that the entire Ordovician ocean-or the part of it which 

formed the mudmound-had a o018 value of -10 per 

mil. This hypothesis is consistent with the 0 18 data of 

Perry and Tan (1972) on marine chert. Perry and Tan 

postulate a change in the oceans of 15 per mil from early 

Precambrian to Holocene. Lowenstam (1961) presented 

evidence for the relative constancy in 0 18 content of the 

oceans from late Mississippian to Holocene. Our data 

suggest that most of the 0 18 change in the oceans since 

early Precambrian took place in the OrdovicianMississippian interval. 

ORIGIN OF THE MUDMOUND 

ORIGIN OF THE ZEBRA LIMESTONE 

By REUBEN JAMES Ross, JR. 

Because the laminated zebra limestone forms the base 

of much of the mudmound one must attempt to understand its origin in order to understand the reasons for the 

mound's existence. Grossly, the striking feature of the 

laminated facies is the seeming alternation of calcilutite 

and radiaxial fibrous calcite (figs. 5, 7). The formation of 

similar calcite layers elsewhere has had widely differing 

interpretations (Bathurst, 1959, p. 511-512; Schwarzacher, 1961, p. 1494-1495; Lees, 1964, p. 518, 523-524; 

Fischer, 1964, p. 114-116, figs. 15, 16F, 17, 18; Ross, 

1972, p. 6-8; Kendall and Tucker, 1973; Jaanusson, 

197 4). In the search for a modern analog of this varvelike 

sediment, we may fail if we look for a modem zebra 

limestone instead of an alternation of two kinds of 

calcilutites, one of which under conditions extant in the 

Ordovician ocean might have changed into radiaxial 

calcite selectively. 
34 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

The origin of the zebra limestone is complex. No simple explanation satisfies all the evidence, much of which 

is contradictory. Any theory of the origin of this striking 

rock must at least take into account: 

1. The great lateral extent and the aggregate thickness 

of the zebra limestone couplets. 

2. The cyclic nature of the laminated rock, involving a 

repetitive stratigraphic sequence of three 

calcilutites, designated ca1, ca2, and ca3. 

3. The determination of whether radiaxial fibrous 

calcite is a cavity filling or a product of neomorphic 

recrystallization. 

4. The position of radiaxial fibrous calcite when present 

always immediately below calcilutite ca1. 

5. The disposition of geopetal sediment, particularly 

peloids, relative to radiaxial fibrous calcite. 

6. Disposition and orientation of fossils within 

calcilutite ca1, sometimes parallel to and 

sometimes normal to the stratification. 

7. Occurrence of calcilutite ca3 (possibly also ca2) as 

seemingly crossbedded "internal sediment" where 

the layers of calcilutite ca1 are disturbed or disrupted. 

8. The drape of zebra limestone without attenuation 

over a "step and riser" in preexisting topography 

and similar physical relationships. 

9. The results of isotopic and other analyses. 

2 

0 

3 

2 

(/) 

UJ 1 _J 

a.. 

~ 0 (/) 

f--

1 

Dolomitl'od 

I 

rI-- I 

I 

Dolomitized 

I 

I 

I 

I 

In attempting to explain the origin of the zebra 

limestone, we have considered the above evidence 

although not necessarily in the order listed. 

GEOCHEMICAL EVIDENCE 

Various analyses were made not only of the laminated 

limestone but also of the more massive part of the 

mound core. We found (1) that all but a trace of insoluble component is in calcilutites ca1 and ca2 rather than 

the radiaxial calcite mosaic, (2) that trace elements are 

somewhat more abundant in the calcilutite than in the 

radiaxial calcite, and (3) that X-ray microprobe analysis 

showed no significant difference between the calcilutite 

and radiaxial calcite in regard to Fe, Mg, and Ca. According to R. C. Surdam, there is no geochemical 

evidence for a hypersaline or supratidal environment for 

either component. 

The isotopic data suggest that the radiaxial calcite 

was deposited in higher salinities or lower temperature 

than the calcilutite in the lowest part of the zebra 

limestone, but both were deposited under the same conditions higher in the mound. This evidence is important 

in eliminating the possibility that origin of the radiaxial 

calcite is allied to percolation of meteoric water as 

Dunham (1969, p. 160-163) proposed for the Permian 

Townsend Mound in New Mexico. It further requires 

that genesis and diagenesis of both radiaxial calcite and 

I I 

Covering beds -

I I 

-

Calcilutite 

I I II -

I I 

~ 7.---------------------------------------------------------.--------------------------, 

Radiaxial calcite 

19.0 20.0 21.0 22.0 23.0 24.0 

FIGURE 36.- Frequency composition diagram for o018 in samples from calcilutite and radiaxial spar of zebra limestone, from mound 

core, and from limestone of covering beds. 
ORIGIN OF THE MUDMOUND 35 

calcilutite have been affected by essentially the same 

factors. 

The analytical evidence therefore favors a totally 

marine origin for the zebra limestone. 

PHYSICAL EVIDENCE 

Most geologists knowledgeable in carbonate sedimentology who have examined hand specimens and thin sections of the zebra limestone from Meiklejohn Peak have 

insisted that the radiaxial fibrous calcite was the 

primary filling of former cavities. Such an interpretation 

would agree with the original inference of Bathurst 

(1959, p. 511; 1971, p. 426-427) concerning the habit of 

radiaxial fibrous calcite. 

However, it has been my belief, based on field 

evidence, that the radiaxial fibrous calcite must have 

replaced some other substance. The distinction between 

the two interpretations could be significant in regard to 

the depth of water in which the zebra limestone was 

deposited. Therefore the evidence is here reviewed. 

More than half the total volume of the zebra limestone 

is composed of radiaxial fibrous calcite (fig. 14). As 

shown in figure 5 (and Ross, 1972, fig. 6) the laminae 

may extend several metres (tens offeet) laterally and are 

present through a stratigraphic thickness of 1-10 m 

(3-30ft). It seems mechanically impossible to maintain 

more than half the volume of the rock as simultaneously 

open galleries in which radiaxial fibrous calcite eventually grew. 

Even if such open galleries could have existed 

simultaneously the stratigraphic evidence calls for 

deposition of calcilutite ca2 in depressions above every 

layer of ca~, followed by growth of a thin tier of radiaxial 

fibrous calcite in virtually every gallery. Water level in 

each gallery would have been very shallow; if it had filled 

the galleries the fibrous calcite would have grown from 

the ceiling as well as from the floor. Above each tier 

peloids of micrite would have been scattered; only then 

could the galleries have been filled with water to their 

ceilings so that radiaxial fibrous calcite could grow 

downward as well as upward. 

The circumstances required to control such water 

levels and sedimentation within a stack of galleries 

would try the ingenuity of a wizard, let alone a 

hydrologic engineer. That the radiaxial fibrous calcite 

now occupies space which was previously simultaneously 

open galleries is here considered impossible. The possibility that such galleries could have been maintained 

open even one at a time is unlikely but conceivable. 

The steplike flexure of the zebra limestone mentioned 

previously would have been structurally beyond belief if 

composed of parallel shells of calcilutite ca, separated by 

parallel layers of water of thickness equal to that of the 

calcilutite. Some substance must have intervened 

between layers of calcilutite ca, to maintain spacing 

between them. 

To the left of the hammer in figure 5 there appears to 

be a lump of calcilutite that has depressed the surrounding laminae. Actually this may be the filling of 

some sort of depression in the laminated sediment but 

that makes little difference. What is important is the 

fact that the seeming lump is completely surrounded by 

radiaxial calcite. 

A similar lump was isolated from a more accessible 

part of the outcrop and sectioned. It was found to be 

suspended in radiaxial calcite in three dimensions. It is 

unlikely that a lump of calcilutite could be suspended so 

that it was completely surrounded by a cavity and that 

the cavity was subsequently filled by radiaxial calcite or 

acicular aragonite. 

I therefore conclude that radiaxial fibrous calcite must 

have replaced some other substance which may have 

been mineral, or vegetable, or partly both. 

Typical cavities filled with alternating generations of 

sparry calcite lining and geopetal sediments are illustrated by Sander (1951, p. 183, fig. 28; p. 184, fig. 29) 

in limestones of the Austrian Alps. Coarse calcite lines 

not only the floor but also the walls and ceiling of the 

cavities. We know of no way to explain precipitation of 

sparry or radiaxial fibrous calcite at selected disconnected spots along the floor, walls, or ceiling of a totally 

submerged cavity. Nor can we explain deposition of 

calcite on the walls or ceiling without deposition on the 

floor of a partially submerged cavity. Yet both circumstances are exemplified at Meiklejohn Peak. 

The large block of laminated limestone from which the 

section shown in figure 7 was taken was variously cut 

and polished to reveal that the radiaxial fibrous calcite 

pinches laterally (figs. 16B, 37) and yet the fossiliferous 

calcilutite layers ca, maintain their same vertical spacing. Where the sparry calcite is lacking, the space is 

filled by a layer of very fine, mostly equigranular 

calcilutite, much of which seemingly exhibits very 

regular foreset bedding. This very fine calcilutite is an 

original layer of sediment in a cyclic sequence (figs. 7, 

11). As shown in figures 16A, 17, 37A, 37B, and 38, parts 

of the very fine calcilutite (ca3) and the bottom of the 

fossiliferous calcilutite (cat) seem to be partly converted 

into radiaxial fibrous calcite. 

In discussing Irish Waulsortian mounds, Lees (1964, p. 

523- 524) attributed the origin of "sheet spars" to the filling of extensive nearly horizontal cracks caused by shear 

failure, but he met with difficulties in explaining how 

roofs of such extensive cavities could be supported. 

Indeed, he questioned how such cavities could even open 

in wet calcareous mud and he appealed to localization of 

layered organic matter to hold the cavities open. 

Commenting on supposed cavities in which the sparry 

calcite of Stromatactis grew, Bathurst (1959, p. 514) 

stated, "the labyrinthine cavity system, supported now 

by such a small volume of intervening siltstone, could 

never have existed in an empty state all at once; it would 
36 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 
ORIGIN OF THE MUDMOUND 37 

have collapsed." After thoroughly discussing possible explanations, Bathurst (1959, p. 520) concluded that "for 

the maintenance of cavity roofs and steep slopes a sediment binder was necessary" and like Lees he considered 

the possibility "that the cavities are, at least in part, 

molds of a buried organism which decayed." 

The position in which we now find radiaxial fibrous 

calcite in many examples is occupied laterally by 

calcilutite ca~ (figs. lOB, 12, 16, 39, 40). Much of this 

calcilutite appears to be crossbedded. In some instances 

the radiaxial calcite appears to be interbedded within 

the crossbeds (figs. lOB, 16, 37A, 38, 39). One might be 

tempted to attribute these relationships to alternating 

calcite lining of the walls of a cavity and internal 

sedimentation, like that clearly illustrated by Bathurst 

(1971, fig. 311) . The superficial resemblance to 

Bathurst's example requires discussion. 

In Bathurst's illustrated example, an unfilled residual 

cavity still exists and the calcite crystals are terminated 

with "dogteeth" pointing into the cavity. Equally important, every layer (or lining) of calcite buried on the floor 

and wall has a continuous counterpart on the ceiling. As 

Bathurst's photograph clearly shows, terminations of 

calcite crystals are preserved even where buried. 

In contrast, no examples of similar-looking calcite 

from the zebra limestone at Meiklejohn Peak had 

pointed terminations (figs. 17, 37A, 37B, 38); we have 

found no open cavities. There are more layers of radiaxial calcite on conjectural floors and walls than on ceilings 

(figs. lOB, 16). There may be a layer of radiaxial calcite 

on a supposed ceiling but not on the corresponding floor, 

and vice versa (figs. 17, 39). Calcite layers within and 

paralleling crossbeds are associated with and grade into 

pelletoid calcilutite; some such layers terminate without 

completely following the contact between crossbeds (fig. 

17). Evidently these examples from the Nevada Ordovician differ significantly from Bathurst's (1971, fig. 311) 

and the radiaxial calcite may be the result of neomorphic 

processes (Folk, 1965, p. 23; Bathurst, 1971, p. 481-503) 

rather than the lining of a cavity. 

We do not suggest that the mudmound of Meiklejohn 

Peak is or was devoid of cavities that became filled by 

spar. On the contrary, we can demonstrate the presence, 

in the upper part of the mound, of "birdseye" and 

similar structures which are conceded to be cavity fillings. But we insist that, within the zebra limestone, 

FIGURE 37. (facing page) - Polished section showing detailed views of 

the same small part of sample illustrated in figure 16B. USGS colin. 

D2325 CO. A, The inner surfaces of the coarse radiaxial calcite (rax) 

lack crystal terminations; such terminations would have been expected if calcite lined the inside of a cavity prior to filling by 

calcilutite ca ~; photographed in polarized light. B, Equigranular 

calcilutite (ca3) appears to have been partly converted to pelletoid 

calcilutite (ca2) as first step toward conversion to radiaxial fibrous 

calcite (rax) ; photographed in ordinary light. 

FIGURE 38. - Coarse calcite along contact at bottom of calcilutite ca, 

is discontinuous and could not have formed lining of a submerged 

cavity prior to filling by seemingly cross bedded calcilutite (ca3). Pelletoid layers, fractures, and contacts may have provided permeable 

channels along which aggrading neomorphism produced radiaxial 

calcite (rax). Note lack of crystal terminations on upper side of lower 

calcite layer (tr). USGS colin. D2325 CO. Collodion peel. 

FIGURE 39. - Coarse calcite (tr) at bottom of crossbedded calcilutite 

ca, is discontinuous; it resembles lower tier (tr) in radiaxial calcite 

(rax) of figure 14B. Radiaxial calcite (rax) above crossbeds is continuous with coarse calcite in pelletoid layers, but is not continuous 

with lower calcite (tr). Therefore, rax, tr, and coarse calcite within 

pelletoid layers did not form the lining of a cavity prior to deposition 

of crossbedded calcilutite ca3• Median seam (ms) is comparable to 

same feature in figures 7, 8, 9, 14B, and 15, and has no relation to 

middle of a preexisting cavity. Presence of calcilutite ca2 filling 

depressions in the upper surface of ca, indicates that calcilutite ca, 

was essentially horizontal prior to deposition of crossbedded 

calcilutite ca3. Radiaxial fibrous calcite may have resulted from 

neomorphic conversion of calcilutite ca3 selectively along pelletoid 

layers. Collodion peel. USGS colin. D2325 CO. 
38 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

cavity filling played a minor role unless such filling took 

place layer by layer accompanied by almost immediate 

lithification of preceding layers. 

ZEBRA LIMESTONE ABOVE BASE OF MOUND 

The previous discussion applies to the zebra limestone 

at the base of the mudmound. That occurrence is not 

unique. High on the south side of the mound a tongue of 

covering rock extends into the mound. As noted 

previously, the mound lithology is re-initiated above this 

tongue by deposition of zebra limestone. As shown in 

figures 6 and 40, the laminae are generally less regular 

and more deformed than in the basal part of the mound. 

Nonetheless the same four lithologic elements are present, listed stratigraphically: 

4. Radiaxial fibrous calcite (rax). Particularly along the 

contact below ca1 and above ca2. 

3. Very fine equigranular calcilutite (ca3) deposited 

above the pelletoid calcilutite. Locally crossbedded. Overlain by another fossiliferous calcilutite 

ca1 . 

2. Pelletoid calcilutite (ca2), filling depressions and 

other irregularities in the top of the fossiliferous 

calcilutite cat. Perhaps somewhat plastic. 

1. Fossiliferous calcilutite (cat), in places much disrupted. Most of the insoluble residue-authigenic 

quartz and potassium feldspar-concentrated in 

this unit. 

The block shown in figure 6 was cut at right angles 

along the line indicated and polished; the resulting surface is shown in figure 40. The fourth and fifth laminae 

from the bottom in figure 40 are of particular interest, 

because seeming conversion of the calcilutite to radiaxial 

fibrous mosaic is incomplete; it can be seen in greater 

detail in figure 13. 

In the thin section (fig. 13) two layers of fossiliferous 

calcilutite ca1 are shown. The lower is overlain by evenly 

bedded pelletoid calcilutite (caz), and that in turn is 

overlain by very fine equigranular calcilutite (ca3). 

However, unit ca3 has been changed partly to radiaxial 

calcite, as has the bottom of the overlying fossiliferous 

calcilutite ca1 in the next cycle. No neomorphism has affected the pelletoid layer. Neomorphism seems to have 

taken place extensively along the upper contact of the 

equigranular calcilutite ca3 and above the median seam 

of the radiaxial calcite (rax). Above the unconverted 

calcilutite ca3 is considerable fragmental material, 

which might be interpreted as fragments loosened from 

the ceiling and fallen to rest on the floor of a cavity. Such 

an interpretation is untenable here; it fails to explain the 

lack of similar material all along the supposed floor to 

the left in this view. Calcilutite ca3 appears to be virtually in place and only partly converted into radiaxial 

fibrous calcite. 

FIGURE 40. - Cyclic calcilutites ca,, ca2, and ca3 in polished surface 

normal to surface shown in figure 6. In fourth and fifth cycles from 

bottom, calcilutite ca3 is incompletely replaced by radiaxial fibrous 

calcite (rax). Thin section illustrated in figure 13 was cut from this 

surface. USGS colin. D2334 CO. 
ORIGIN OF THE MUDMOUND 39 

Figure 41, another polished section, shows lithologic 

relations 7 mm from the surface shown in figure 40. The 

same features are present except that in figure 41 the two 

patches of coarse calcite ( tr) in the lower part of the very 

fine equigranular calcilutite have coalesced. Two 

enlarged views in polarized light of this polished mirror 

image are instructive. Figure 18A shows the full 

thickness of equigranular calcilutite ca3 completely converted to radiaxial calcite (rax) on the right and divided 

into upper and lower halves by the smooth median seam. 

To the left the median seam is above the unconverted 

calcilutite; in this sector the crystals of spar are large 

above the seam and only very small below. This sector 

magnified still further is shown in figure 18B. 

Figure 18B presents an informative puzzle. If we assume that the radiaxial calcite is filling a cavity, the meFIGURE 41. - Two patches of the same radiaxial calcite as shown in 

figure 13 and figure 40 have coalesced below calcilutite ca3 in a distance of 7 mm. Polished surface, facing that shown in figure 40 at a 

distance of 7 mm. USGS colin. D2334 CO. 

dian seam should be the surface along which crystals 

growing from floor and ceiling met. As discussed 

previously, we may also assume that dogtooth calcite 

crystals face the inside of a cavity. The radiaxial calcite 

above ca3 seems to have such teeth pointing downward 

into the calcilutite; we therefore might suppose that 

calcilutite ca3 filled a cavity of which the radiaxial 

calcite immediately above formed the ceiling. But this 

same calcite, we have already noted, should have been 

growing upward to close a cavity at the median seam. 

How radiaxial calcite crystals could have remained 

suspended in space to grow both upward and downward 

to form the floor of one cavity and the ceiling of another 

is difficult to comprehend. 

Therefore, I suggest that the equigrains of calcilutite 

ca3 began to aggrade to form increasingly larger crystals 

upward toward the median seam. This was no cavity 

floor. The median seam is jagged here; it would have 

become smooth when crystals above and below grew to 

the same size. Calcilutite ca3 was being selectively converted to radiaxial fibrous calcite. 

PELLETOID CALCILUTITE 

Beales (1958, p. 1867-1871) reviewed the occurrence of 

"Bahaman type" limestone characterized by its pelletoid texture and he favored a nonskeletal aggregation 

of finely crystalline aragonite or calcite in agitated and 

possibly supersaturated water to form such sediments. 

Whether the pelletoid material within the Meiklejohn 

Peak zebra limestone was derived from fecal pellets or 

from mechanically aggregated microcrystalline carbonate or is a result of structure grumeleuse (Cayeux, 

1935, p. 271-272) is uncertain, but a case could be made 

for each of these origins. The correct interpretation bears 

on the origin of radiaxial fibrous calcite. 

In figure 39, two layers of calcilutite ca1 are separated 

by what appears to be crossbedded pelletoid calcilutite 

and homogeneous calcilutite. A tier of calcite (tr) 

separates the bottom of the cross bedded calcilutite from 

ca1 but pinches out to the left; scattered small depressions in the top of ca1 are filled with another calcilutite 

(ca2) beneath the tier of calcite. Above the crossbedded 

calcilutite is a small body of radiaxial fibrous calcite 

which grades downward into two of the crossbeds; these 

two crossbeds are pelletoid, but they are so impregnated 

with radiaxial calcite that there is no clear boundary 

between the calcite and the pelletoid calcilutite. 

The radiaxial calcite at the bottom is not the same as 

that on top; if it were, both would grade into the crossbeds. The lower one clearly does not. Therefore, there 

was no submerged cavity between the two layers ca1 at 

the time the lower radiaxial calcite was formed or it 

would have formed continuously and simultaneously 

along the floor and on the ceiling. 

If the pelletoid layers were originally pelletoid their 
40 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

high permeability may have permitted easy flow of 

supersaturated water, in the same manner as contacts 

between the layer. Increased permeability may have 

stimulated the formation of radiaxial fibrous calcite in 

selected parts of the sediment. 

On the other hand, some unknown factor, perhaps increased permeability along sedimentary contacts, may 

have enhance the conversion of homogeneous calcilutite 

to grumeleuse calcilutite as an intermediate step in the 

change to radiaxial fibrous calcite. 

In figure 17, radiaxial fibrous calcite is somehow intimately related to pelletoid material. And in figure 38 

there is a suggestion that pieces of calcilutite considerably larger than pellet-size are in the process of being surrounded by or perhaps converted to calcite. 

Two even more detailed views, figures lOB and lOC, 

also could be interpreted to show that radiaxial calcite is 

formed with the assistance of or at the expense of pelletoid material. These two photomicrographs show parts 

of the thick section shown in figure lOA. 

Shinn (1969, p. 133, figs. 21 , 22) observed that the pelletoid texture of a modern carbonate mud may be accentuated by inward replacement or recrystallization of the 

periphery of each original peloid. This observation suggests to us that a similar mechanism could modify any 

calcilutite which had been divided into pellet-sized particles or aggregates. 

AGGRADING NEOMORPHISM: OTHER VIEWS 

As early as 1904 Cullis concluded that aragonitic mud 

and fossil shells had been converted to calcite spar at 

Funafuti atoll, Ellice Islands; this evidence has been 

reviewed by Bathurst (1971; p. 350-355). Bathurst (1971, 

p. 489- 490) also recorded the discovery that aragonite 

shells may be replaced by fibrous sparry aragonite. 

Kendall and Tucker (1973) carefully reviewed the 

evidence on the formation of radiaxial fibrous calcite and 

concluded that it is a product of replacement of acicular 

metastable carbonate and that replacement took place 

by a process of solution-precipitation and by "migration 

of a fluid film through the acicular host." According to 

this theory the acicular carbonate filled voids prior to 

replacement. 

Ross, Friedman, and Jaanusson (1971) considered that 

radiaxial fibrous calcite could be a product of selective 

recrystallization of carbonate mud in a totally marine 

environment. The process they envisaged was similar to 

that of Kendall and Tucker but was independent of a 

preexisting acicular cement. 

EVIDENCE OF RUPTURE STRUCTURES 

Although the spacing of laminae is remarkably uniform there are evidences of minor deformation, rupture 

of layers, and flexures irregularly throughout the facies 

(fig. 5). To examine an example of such disruption the 

block of limestone shown in figure 10 was cut serially, 

sectioned, polished, and etched for collodion peels. As 

already noted, there are striking differences from as well 

as strong similarities to, the section shown in figure 7. 

(The two sections occurred within 60 em of each other.) 

One section of the disrupted rock (fig. lOC) is particularly enigmatic. 

In most of the subparallel laminae of the zebra 

limestone, fragments of fossils which characterize the 

calcilutite ca t are oriented randomly or roughly parallel 

to the laminations (figs. 9, 11) as might be expected. 

However, where the the calcilutite ca1 is deformed into 

flexures and( or) ruptured, the layering in calcilutites ca2 

and ca3 appears crossbedded and the fossil fragments in 

calcilutite layers cat may be oriented at angles as high as 

90° to the lamination (figs. lOB, lOC, 12). It seems 

patently impossible to devise a mechanism by which 

these laminae could have been deposited on edge as the 

orientation of fossils seems to require. Strangely, these 

discordant orientations are not limited to the small areas 

of deformed laminae and are independent of the seeming 

crossbedding of ca2 and ca3. 

Moreover, a shell of an inarticulate brachiopod still 

partly enclosed in calcilutite (fig. lOC) projects into a 

combination of radiaxial fibrous calcite and pelletoid 

crossbeds. How could this fossil have survived any 

mechanical separation of layers of calcilutite cat? 

We may suppose that the layers of calcilutite cat were 

all part of a single body, that it was cut by subparallel 

tensional fractures which resulted in cavities, and that 

these cavities were filled by semifluid, partly pelletoid 

calcilutite. But if there were any component of shear, as 

suggested by Lees (1964, p. 523-524), the fossil should 

have been broken. Indeed, one wonders how the fossil 

could have avoided breaking unless the intervening pelletoid material acted truly as a fluid of low viscosity. 

On the other hand we may be dealing with cyclically 

deposited sedimentary layers one of which (ca2 and( or) 

ca3 ) has been partly and preferentially replaced by or 

recrystallized into radiaxial fibrous calcite in each cycle. 

Originally, this layer may have behaved much like a 

fluid because of high water content, while calcilutite ca1 

was more cohesive and coherent because of its organic 

content. 

Philip N. Playford kindly examined examples of the 

zebra limestone and pointed out (oral commun., April 

1972) that the accordance in attitude of the supposed 

crossbeds (figs. lOC, 12) might indicate that spaces 

between many layers of calcilutite ca1 were open at the 

same time, canted at a steep angle to the horizontal, and 

filled by a succession of nearly horizontal pelletoid and 

equigranular calcilutite layers; he further suggested that 

key layers found in a succession of spaces could confirm 

this interpretation. Key horizons cannot be traced 

precisely from one set of " crossbeds" to the next and 
ORIGIN OF THE MUDMOUND 41 

there is no simple sedimentary explanation for the three 

divergent contradictory orientations of (1) layers of 

calcilutite ca1, (2) positions of fossils within yet normal to 

layers of ca1, and (3) crossbedded calcilutite ca3 • 

We have considered the possibility that layers of 

calcilutite ca1 might have become indurated and 

buckled in a manner similar to that deduced from Ordovician limestone in the Baltic area by Lindstrom 

(1963, p. 252-256) or similar to the cemented layers 

forming today in the Persian Gulf (Shinn, 1969, p. 

112-119, 122, 128). Buckling of such layers involves shear 

and we have already discounted any explanation involv 

ing shear. 

Although we do not know how the sediment was 

deposited, we propose that the calcilutites ca2 and ca3 

might have behaved thixotropically because of high 

water content. Perhaps hydraulic pressure contributed 

locally to the rupture of calcilutite ca1 and more broadly 

to its support. Distortion and rupturing of layers, as illustrated in figures 6, 10, and 40, may have occurred in 

such a fluid environment, wherein unconsolidated thixotropic calcareous muds (ca2 and ca3) flowed through 

ruptures in more competent ca 1 to mix with other unconsolidated mud in younger or older cycles. 

This theory eliminates the necessity for mechanically 

impossible open galleries equaling 60 percent of the final 

volume of the rock. It provides for local disruption of 

some layers and the undisturbed nature of others. It accounts for flowage of internal sediment, permitting as 

much upward as downward movement. 

Accordingly, each layer of fossiliferous calcilutite ca 1 

may be considered as the more competent layer in a 

repeated cycle and may represent a time of slow 

sedimentation and of partial winnowing of the calcilutite 

to concentrate fossil remains. Layers of pelletoid and 

homogeneous calcilutite (ca2 and ca3) may have been 

deposited over a much shorter time and in more turbulent water. The unconsolidated and saturated mud 

(ca2 and ca3) could have provided the permeable avenues 

wherein aggradational neomorphism eventually converted the calcilutite to radiaxial fibrous calcite. This 

conversion may have taken place selectively along certain sedimentary contacts. Whether pelletoid muds contributed to permeability and easier access of CaC03-

bearing water or were the result of neomorphic production of structure grumeleuse is uncertain. In either 

process, rapid crystallization, probably of aragonite, 

within the saturated, unconsolidated carbonate mud 

was necessary to preserve the cyclic nature of the zebra 

limestone. 

RAPID SUBMARINE LITHIFICATION 

Alternatively, one may speculate that rapid cementation or aggrading neomorphism contributed to the 

production of successive hard, lithified sedimentary 

crusts a few millimetres thick and that each crust expanded laterally as it lithified, according to Shinn's 

(1969, p. 128, fig. 17) observations in the Persian Gulf. As 

a result, each new crust may have had the strength 

necessary to hold itself off the underlying crust with a 

few points of contact. In the resulting slim cavities 

calcite or aragonite may have crystallized from saturated 

seawater, or pelletoid and fine homogeneous carbonate 

mud may have been introduced from the overlying sea 

bottom. 

DERIVATION FROM ALGAL MATS 

In describing Alpine Triassic zebra limestone, very 

similar to the Ordovician laminated limestone from 

Meiklejohn Peak, Fischer (1964, p. 127) noted their close 

resemblance to modern algal mat deposition. Such mat 

deposits come most readily to mind when we seek a 

modern laminated analog. 

Direct evidence of algae within the carbonate mound 

is scarce. Algae are present but those few that are 

preserved could hardly have acted as the binder for the 

entire mound. Sphaerocodium (fig. 42) is present in collections D2349 CO, D2348 CO, D2343 CO (fig. 4) as scattered filaments. Nowhere does it form the closely packed 

structures reported in Devonian reefs of Australia 

(Wray, 1967, p. 35- 40) and Canada (Wray and Playford, 

1970, p. 547-552, pl. 2, figs. 1-4). 

The problematic micro-organism Nuia is found in the 

calcarenite parts below and in the mound in collections 

D2341 CO (echinodermal lime packstone), D2345 CO, 

D2346 CO, D2347 CO, D2348 CO, and D2349 CO (fig. 4). 

This form was reported by Toomey (1970, p. 1325, fig. 

12a) and by Toomey and Klement (1966) in Lower Ordovician carbonate mounds of the El Paso Limestone, 

west Texas. At Meiklejohn Peak Nuia is a minor constituent and seems to occur in fragmental deposits 

within the mound. In fact Nuia could not have been 

significant in binding the mound together. 

Rowell and Krause (1972, 1973) inferred that plants, 

presumably algae, were present on the mound, because 

two genera of inarticulate brachiopods were adapted for 

attachment to "cylindrical objects." 

Although we have no direct evidence that algae were 

abundant enough to form extensive mats, we must not 

completely exclude the possible former existence of 

green or blue-green algae, all remains of which have disappeared. 

G. M. Friedman and others (1973) showed that carbonate laminites may have been deposited within algal 

mats in the geologic past, and they suggested a 

mechanism for the formation of calcite cement in layers 

within the mats themselves. Their observations support 

the belief that algae not only may trap and hold 

sedimentary particles but also may be directly responsible for the "precipitation of carbonate laminites which 
42 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

preserve the morphology of the mats even after the 

organic matter has become degraded and has disappeared" (Friedman and others, 1973, p. 552). 

Such a process, if cyclic, could result in formation of 

zebra limestone one lamina at a time without requiring 

any voids. Furthermore, irregularities in the bottom configuration would be duplicated by the mats; damage to 

mats would result in depressed features like those shown 

in figure 5. Local upward disruption of laminae could 

have resulted in structures shown in figures 6 and 10. 

The trapping of small fossils on edge (fig. 10C) within an 

algal mat is conceivable. Without such a mat it is nearly 

impossible to explain why such fossils are not parallel to 

the stratification. 

Otte and Parks (1963, p. 394) concluded that radiaxial 

fibrous calcite forming Stromatactis-like masses in Upper Pennsylvanian and Lower Permian limestone in New 

Mexico had taken the place of a firm-bodied organism 

capable of supporting mud layers but incapable of 

secreting a calcareous skeleton. Some modern blue-green 

algae have the consistency of tough rubber. Layers of 

such a rubbery organic material might have supported 

FIGURE 42. - Filaments of Sphaerocodium found in main core of 

mudmound, particularly at top. Thin section. USGS colin. D2348 

co. 

layers of calcilutite and later been replaced by sparry 

aragonite. Perhaps, one need no longer speculate on this 

sort of origin for the radiaxial calcite, inasmuch as G. M. 

Friedman and coauthors (1973) have observed a 

reasonable analog. 

The zebra limestone might have formed on a tidal flat 

but Hardie and Ginsburg (1971) found that cyclic 

laminae on Bahaman tidal flats are formed during 

periods of major storms, not as the result of diurnal 

tides. They further noted that persistent winds, over 30 

km/hr, "are needed to suspend pelleted lime mud * * * 

and flood the tidal flats with sediment-charged 

seawater. Deposition from these floodwaters produces 

either a thin mud lamina (made by the flypaper-like 

trapping action of algal mats) or a couplet consisting of a 

basal mud layer overlain by a sorted layer of fine sand 

peloids." 

Like the couplets observed by Hardie and Ginsburg on 

Andros Island, Bahama Islands, the laminae at Meiklejohn Peak are composed of a basal mud (ca1) (rich in ostracodes and other fossils) and a layer of well-sorted pelletoid mud (caz). But a third layer of calcilutite is present in the cycle; where laminae lie flat, this third 

calcilutite (ca3) is equigranular, but where laminae are 

disrupted or deformed, calcilutite ca3 appears pelletoid 

and crossbedded (fig. 38, 39). 

In Shark Bay, Western Australia, well-laminated sediments are, according to Davies (1970, p. 183-186), only 

found with smooth algal mats and they occur in two environments- in the outer intertidal zone and in the 

landward parts along the floors of tidal channels. 

Neither dolomite nor gypsum is present in sediments in 

the intertidal zone, but both are present, in varying 

amounts, in the supratidal zone (Davies, 1970, p. 177, 

table 1). 

The laminated rock at Meiklejohn Peak lacks any evident dolomite, magnesian calcite, or gypsum. Therefore, 

deposition in the intertidal, rather than the supratidal, 

zone is favored. 

The small "cabbagehead" masses mentioned 

previously (p. 13) (fig. 19) may have resulted from 

deformed growth of algal mat over preexisting 

topographic irregularities (compare with Davies, 1970, 

figs. 12 B, C). Similarly, deformation of calcilutite ca1 as 

shown in figure 10 may have been caused by breaching of 

such a " cabbagehead," by upward movement of fluid 

mud or of gas (Davies, 1970, p. 191- 192), or by bioturbation. 

EFFECT OF ORGANIC SUBSTANCES 

Recently, Meyers and Quinn (1971, p. 992) showed 

that fatty acids and other lipids are absorbed by calcite 

in seawater and may provide a coating that prevents the 

solution of calcite in seawater which is undersaturated 

for calcite. The coating may also prevent the precipita-
ORIGIN OF THE MUDMOUND 43 

tion of calcite in supersaturated seawater. It therefore 

seems to us possible that the presence or absence of such 

organic substances within some of the components of the 

laminae at Meiklejohn Peak could have influenced the 

formation of sparry calcite, although identifying that influence now may be virtually impossible. 

The important influence of blue-green algae, other 

organisms, and organically derived chemicals in the formation of modern laminated carbonate sediments in the 

Gulf of Aqaba has been demonstrated by Friedman and 

others (1973, p. 553-556). These same authors (p. 552, 

figs. 9, 14) showed not only that the algal mat encloses 

great amounts of particulate carbonate but also that the 

laminites may consist of alternating layers of "cryptocrystalline high magnesian calcite and fibrous 

aragonite." Such alternations are suggestive of the cyclic 

calcilutites and radiaxial fibrous calcite of the zebra 

limestone. 

Bathurst (1967) called attention to a "subtidal 

gelatinous mat" which acts to stabilize the calcareous 

sands of the Great Bahamas Bank. This mat is 

widespread. The mat seems to be partly algal but it is inhabited by and is the food source for such a multitude of 

minute animals that its precise origin is a mystery. According to Bathurst, such a mat could leave no direct 

evidence in the geologic record, for it disappears as soon 

as buried. And yet if this mat were suddenly buried 

without disruption by very fine sediment, we believe an 

organic film would surely be trapped. 

Zangerl (1971) reviewed the effect of decaying animal 

matter in the formation of concretions, involving the 

rapid crystallization of calcium or other carbonates. 

There should be little doubt that organic substances 

may exert influences on the deposition of carbonate sediments, influences so complex and so little understood 

that current conventional thinking about the origin of 

limestone is inadequate. 

THEORY OF LIFTING BY CRYSTAL GROWTH 

Several of our colleagues suggested that the sparry and 

radiaxial calcite may have grown in place, "shouldering" 

its way along the contact between couplets; they 

proposed that the growth of the calcite, like the growth of 

gypsum and trona in the Pennsylvanian Paradox 

Member of the Hermosa Formation and in the Eocene 

Green River Formation (Deardorff and Mannion~ 1971, 

p. 35; 0. B. Raup, R. J. Hite, and W. C. Culbertson, oral 

commun., 1972) might have lifted the overlying layers as 

it grew-the displacive precipitation theory of Folk 

(1965, p. 24). In many of the Ordovician laminae there 

are pelletoid bodies within the lower layers of the radiaxial fibrous spar (fig. 8); these bodies might be compared 

with fragments of the underlying shale which have been 

lifted during crystal growth into the coarsely crystalline 

gypsum or trona of the Pennsylvanian and Eocene 

deposits. However, at Meiklejohn Peak where the 

radiaxial fibrous calcite layer is incompletely formed 

(fig. 7), the calcilutite couplets obviously were not 

mechanically disturbed by formation of the calcite. In 

such places the radiaxial calcite appears to have grown 

at the selective expense of one or more of the calcilutites. 

CONCLUSIONS 

The original sediment of the zebra limestone was 

probably an alternation of partly or wholly cemented fossiliferous calcilutite with unconsolidated saturated carbonate mud. Much of the latter was pelleted. Whether 

pelleted muds were produced as fecal pellets or whether 

they were in some way related to algal mats is unknown. 

No direct evidence for such algal origin was found. In 

either circumstance pelleted texture would have 

produced high permeability, thereby aiding the diffusion 

of seawater saturated with CaCOa. 

The depth of water is uncertain. If the formation of the 

laminae was related to algal mats one might expect very 

shallow, even supratidal, conditions to have been possible. Isotopic analysis indicates that all components of 

the mound were formed under the same conditions, 

precluding vadose water as an agent of diagenesis and 

making unlikely a supratidal environment for formation 

of the radiaxial fibrous calcite in the zebra limestone. 

Inasmuch as the main core of the carbonate mound 

rests upon the zebra limestone, growth of the mound 

may have started in depths not exceeding 30 m. 

Shinn (1969, p. 110, 112-117) showed that submarine 

cementation can take place in depths of 30 m in the 

Persian Gulf although examples in which he found multiple crusts of relatively close vertical spacing were at 

depths of less than 3 m. He also found that submarine cementation took place in areas of low sedimentary accumulation such as the windward side of offshore 

highs. Presumably the essential factors of low sediment 

accumulation, saturated seawater, and permeable sediment could be found at considerable depths. 

We do not know what, if any, influence algae, sponges, 

or other organisms had in protecting the depositional 

surface, in binding the sediment, or in controlling 

crystallization of CaCOa. Although Meiklejohn Peak 

must have been scores of miles from the shore, it surely 

was not subjected to wave action sufficiently destructive 

to tear apart the laminated zebra limestone. 

The fact that cephalopod shells are most common in 

the laminated facies, particularly in pockets, suggests 

that their empty shells, weighted down by cameral 

deposits and waterlogged, were swashed into shallow 

depressions and cemented into the surrounding rock. 

Ginsburg and James (1973, p. 24) have discovered off 

the coast of British Honduras that lithification of 

limestone has taken place at depths not less than 

20 m in completely submarine conditions; one 
44 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

specimen was found to be less than 2,500 years old. They 

have found that the cement is aragonite and highmagnesium calcite, both of which are unstable minerals. 

The cement, according to Ginsburg and others (1973, p. 

781), "almost obliterates many primary depositional 

fabrics." If such is the case in the Neogene off British 

Honduras we can speculate that the inversion of the cements to calcite will unavoidably result in further 

recrystallization and obliteration of original sedimentary 

fabric. 

Ginsburg and James' discovery of cavities filled or 

lined with "dense, radiating fibrous growths of 

aragonite" provides a lithology which could readily invert to the radiaxial fibrous calcite of the zebra 

limestone and the Stromatactis-like bodies of the main 

mudmound at Meiklejohn Peak. 

Despite our lack of positive evidence for algal mats, 

the observations of G. M. Friedman and others (1973) 

suggest that the origin of the zebra limestone should be 

attributed to mats. Most fossils in calcilutite ca1 are 

oriented parallel to laminations where layers are undisturbed but the shells lie at angles as high as 90° to 

laminations, particularly where layers are disturbed; 

each lamina was deposited initially as a layer draped 

over various irregular surfaces where fossils settled and 

were trapped in the cohesive "flypaper" of algal filaments. 

By no analogy other than to algal mats can we as 

easily explain the great lateral extent of laminae in the 

zebra limestone, the complete isolation of lumps of 

calcilutite, and the steplike flexure of laminae. Seasonal 

climatic fluctuations-whether producing storms, 

changes in water chemistry, or alternations of algal 

growth-could have accounted for the varvelike cyclic 

nature of the calcilutites ca~, ca2, and ca3. The radiaxial 

fibrous calcite may have been a product of selective 

recrystallization of aragonite needles within the algal 

mat, or of micritic metastable carbonate within or below 

the mat, or both. 

One of the calcilutites, possibly ca~, may represent the 

micritic material within the algal mat itself and 

therefore may appear to have been the most competent 

layer in the cycle. Locally, ca1 was deformed just as we 

might expect an algal mat to be where ruptured by 

bioturbation, wave action, or the upward pressure of gas 

or contained fluid. 

Locally, there must have been a briefly fluid, possibly 

thixotropic, pelletoid calcilutite ca2 and calcilutite ca3 

between layers of ca1; whether the resulting carbonate 

slurry filled cavities from above or was deposited as a 

layer in a repeated cycle and was locally squeezed 

hydraulically into flow structures cannot be proved. 

The zebra limestone seems to indicate that intertidal 

or extremely shallow infratidal deposition initiated the 

building of the carbonate mudmound and Meiklejohn 

Peak. However, the depth to which algal mats may extend depends largely on the abundance and voracity of 

browsing marine fauna. 

The occurrence of minor amounts of zebra limestone 

above tongues of cover beds, approximately 25 m above 

the bottom of the mound, proves that for the first third 

of its growth the mound was only slightly above the surrounding bottom. We must therefore conclude that the 

covering silty limestone of the Orthidiella zone was 

likewise deposited in shallow water in the vicinity of 

present-day Meiklejohn Peak. 

Although some radiaxial fibrous calcite may occupy 

former cavities it may also have resulted from recrystallization and neomorphic aggradation of other metastable 

carbonate. Whether triggered by organic substances, by 

the instability of aragonite, or by greater permeability, 

this neomorphism was highly selective. 

Further, it must have taken place very early in the 

diagenetic process, perhaps as rapidly as the algae 

de com posed. 

These conclusions free us from the necessity of 

postulating the prior existence of cement-filled cavities 

wherever radiaxial fibrous calcite is encountered. We 

need not pursue an ontological speculation on the 

critical size of a cavity. 

If, as Kendall and Tucker (1973) believe, radiaxial 

fibrous calcite is derived by recrystallization from 

acicular aragonite, why should it not be derived even 

more readily from micritic aragonite or high-magnesian 

calcite? If such a fine-grained aragonite were converted 

by aggrading neomorphism to coarse radiaxial spar the 

oldest crystals - those that increased most by incorporating their neighbors - should be the largest (figs. 

14C, 18B). Bathurst (1958, p. 15, 20; 1959, p. 509) 

showed that grain (crystal) size of calcite increases with 

growth away from the wall of the cavity; that is, the 

older crystals are the smaller. I do not dispute this 

finding, but I question the belief that direction of increase in grain size always indicates the center of a 

former cavity. The direction of neomorphic conversion of 

micrite would be from coarse (old) to fine (young) 

crystals, a very different conclusion involving no cavity 

at all. Neomorphic conversion can be demonstrated at 

Meiklejohn Peak; the conversion of an echinodermal 

plate to radiaxial calcite is shown in figure 29. Calcite 

and fossils left relict in radiaxial fibrous calcite are illustrated in figures 30 and 31. 

I suggest that the median seam, which is virtually ubiquitous in such calcite, does not represent the surface 

along which crystals grown from opposing ceiling and 

floor have met. The seam may be entirely related to 

recrystallization, being some sort of surface of equal 

pressure between two tiers of large aggrading crystals. 
ORIGIN OF THE MUDMOUND 45 

ORIGIN OF THE CORE OF THE MUDMOUND 

By V ALDAR J AANUSSON 

The Meiklejohn Peak mound had no skeletal frame. 

Constituents of the skeletal sand do not indicate the 

presence of organisms that could have acted as important sediment traps. Sponges are a possible exception, 

provided that the spiculaelike skeletal particles belong 

to this group. Nevertheless, the "spiculae" do not seem 

sufficiently numerous, and macroscopic skeletons of 

sponges are very rare. The prolific occurrence of sponges 

(and Pyritonema) in the so-called sponge beds of the Ikes 

Canyon section, Toquima Range, Nevada, is not associated with a mound. Whatever organisms did control 

the growth of the mound must have lacked preservable 

hard parts. 

M. J. Brady (oral commun., Nov. 1971; written commun., April3, 1972) found that low banks of lime mud in 

lagoons of Yucatan are not dependent on any framework 

organisms. The buildup of mud is related to hydrologic 

conditions- to changing currents, to bottom configuration, to wind direction, and to tidal currents. As soon as 

the mudbanks "became prominences on the lagoon 

floor" they seemingly grow, because they disrupted 

sediment-laden currents. 

Understanding the original nature of the carbonate 

mound requires interpretation of how Stromatactis 

formed. These structures may occupy 50 percent or more 

of the volume of a carbonate mound. Even at the Meiklejohn Peak mound they form 15-20 percent of the volume. 

Bathurst ( 1959) suggested that Stromatactis represents 

cavity-fillings - a proposal accepted by most subsequent writers who investigated such structures. This 

suggestion implies that during growth or diagenetic 

history of a mound a considerable volume was occupied 

by cavities of macroscopic size. That Stromatactis in 

fact is cement precipitated into cavities can be proved in 

many instances but this does not necessarily mean that 

all Stromatactis-like calcite bodies have the same origin. 

Such calcite bodies can originate through recrystallization although bodies so formed lack a median seam. 

Moreover, radiaxial mosaic forming Stromatactis can 

demonstrably grow at the expense of included sedimentary grains and surrounding sediment. The quantitative 

importance of the latter process, though not always evident, seems to be small in comparison with the volume 

of a Stromatactis. The evidence from the Meiklejohn 

Peak mound is not conclusive but if the median seam of 

radiaxial mosaic does indicate cavity-filling, then most 

of the Stromatactis in the mound has been formed as cement precipitated into cavities. 

Interpretation of the growth of the mound depends on 

that of the diagenesis in the early generations of 

limestone. If in the Meiklejohn Peak mound the 

generation Li 1 (fig. 32) was bored by organisms-if it 

was lithified prior to the deposition of Li 2 -lithification 

of parts of the mound took place early enough for formation of hardgrounds. In that case, the mound probably 

represented a lithoherm (as defined by Neumann and 

others, 1972) and not a mound of carbonate mud. In fact, 

much of what now looks like a lithified carbonate mud 

would possibly not have been soft sediment at all but a 

rock largely formed by calcium carbonate micrite 

precipitated from the seawater. Recent limestone, 

formed largely by precipitation of high-magnesian 

calcite, has been described from the Mediterranean 

(Alexandersson, 1969). Some of the rock from the 

lithoherms of the Straits of Florida (Neumann and 

others, 1972) is closely similar to the mound at Meiklejohn Peak in appearance (my observations on the 

material exhibited by A. C. Neumann at the AAPG 

meeting in Denver, April 1972) as well as composition. 

This type of rock is termed here "thalassic limestone." 

Particles of sediment were trapped and encrusting 

organisms were incorporated therein, but it formed 

mainly by precipitation of calcium carbonate from 

seawater. The growth of thalassic limestone tends to be 

irregular. Cavities of various shape and size can be 

enclosed within the rock and filled by trapped sediment 

or drusy cement, or both. The organisms associated with 

modern thalassic limestones are mainly those that bore 

into the rock or attach themselves upon the surface of 

the rock, that is, boring endofauna and sedentary 

epifauna. Such rock tends to be intensely bored by 

various organisms (Alexandersson, 1969). 

If the Meiklejohn Peak mound and similar carbonate 

mounds were once lithoherms, how the large volume of 

cavities could be formed in the mound and could exist 

without collapsing would be easily explained. The early 

formation of spar in Stromatactis would also be easy to 

understand. More difficult to explain would be the 

locally distinct lineation parallel to the depositional surface of the former cavities within the mound. 

During field examination of the Meiklejohn Peak 

mound we considered the possibility that the mound 

represented a lithoherm. However, no structures could 

be recognized that would indicate the former presence of 

hardgrounds. No bored surfaces were found, or even any 

unmistakable traces of boring activities. The presence of 

several generations of limestone was discovered later, 

during laboratory examination, in some samples. 

Skeletal remains of encrusting organisms are rare in the 

macrofauna of the mound. However, on Ordovician 

hardgrounds (discontinuity surfaces) an encrusting 

epifauna is generally notoriously poorly represented 

(Jaanusson, 1961). It is also important to note that when 

the growth of a Stromatactis carbonate mound had 
46 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 

ceased, the mound was demonstrably lithified and 

formed a hardground before covering beds were 

deposited. This cannot be proved with respect to the 

Meiklejohn Peak mound, but it is true for numerous 

other Paleozoic carbonate mounds. At least in the Ordovician carbonate mounds of Sweden, the mounds' upper surfaces show no evidence of borings; environmental 

conditions obviously were not suitable for boring 

organisms. An epifauna encrusting the surface has not 

been observed. 

If the Meiklejohn Peak mound was not a lithoherm, it 

presumably was soft carbonate mud which lithified early 

but not at the interface between sediment and seawater. 

If the mound had emerged before deposition of the covering beds, the cavities could have formed as a result of 

differential shrinkage within the mound during subaerial 

exposure. Emersion and shrinkage could have been followed by internal sedimentation, lithification, and 

cementation in the cavities (Dunham, 1969). However, 

contact relations between the mound and the covering 

beds do not suggest a phase of emersion between these 

two periods of deposition. Moreover, the data on o018 indicate that the spar and calcilutite have the same 

isotopic composition and that precipitation of the spar 

from fresh water is very unlikely (p. 31). 

It is difficult to find a suitable actuogeological model 

for the Meiklejohn Peak mound if formation of the extensive cavities and early lithification took place within 

the carbonate mud in constantly submarine conditions, 

as postulated by many writers for similar carbonate 

mounds (Bathurst, 1959; Schwarzacher, 1961; Lees, 

1964; Heckel, 1972, and others). The origin of the 

cavities has been variously explained: space left after 

decay of soft organisms (Bathurst, 1959; Lees, 1964), differential settling and shear failure (Schwarzacher, 1961), 

solution (Textoris and Carozzi, 1964), burrows made by 

animals (Shinn, 1968), and differential compaction 

(Heckel, 1972). Some of the explanations (solution, burrows) are very unlikely for the Meiklejohn Peak mound 

and for other mounds herein mentioned. There is no 

positive or negative evidence for the former existence of 

abundant soft organisms whose decay could have left the 

cavities after the enclosing carbonate mud had become 

firm (see also Heckel, 1972). Shear failure alone cannot 

be an important factor in forming the cavities, because it 

implies that the mound was originally much larger. Differential settling and differential compaction, or a combination of both, are possible agencies in formation of 

cavities within a soft sediment but our knowledge is 

scanty about what really happens when a mound consisting of carbonate mud settles and compacts. Small 

cavities of the general shape of microstromatactis have 

been experimentally produced by settling Holocene carbonate mud (Cloud, 1960, 1962). However, whether the 

same or a similar process can produce macroscopic 

cavities aggregating as much as 50 percent of the total 

volume of a carbonate mud is not known. Differential 

settlement of a mound has demonstrably caused 

transverse fissures and crevices but formation of a considerable volume of horizontal cavities is not readily explained from the standpoint of soil mechanics. Thus, if 

the Meiklejohn Peak mound was once a mound of soft 

carbonate mud continuously in a submarine environment, there is much in the diagenetic history of the 

mound that we do not understand. 

The depth of water during formation of the Meiklejohn Peak mound is difficult to determine. The presence 

of calcareous algae suggests deposition within the 

general limits of the photic zone. The bedded limestone 

covering the mound abounds in what probably have 

been indurated pellets. In modem seas such pellets are 

not known to be formed below the upper photic zone, 

and they may indicate similar shallow-water conditions 

during the deposition of the covering beds. Regardless of 

whether the Meiklejohn Peak mound was or was not a 

lithoherm, the calcarenitic covering beds give the 

impression of having been deposited in an environment 

with a higher average water energy level than that which 

prevailed during deposition of the soft carbonate mud of 

the mound. The significance of this difference is not 

clear; in carbonate sediments estimating the water 

energy level from the grain size distribution is usually 

difficult mainly because of the trapping and binding action of plants (Ginsburg and Lowenstam, 1958; Baars, 

1963; Lynts, 1966; Scoffin, 1970) and the local occurrence of gelatinous mats on the surface of the sediment 

(Bathurst, 1967). The aspects related to depth of deposition can be discussed with greater confidence when the 

carbonate lithology of contemporaneous beds has been 

studied in a regional scale. 

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