Primary Textures in 

Experimentally Formed 

Oolite Cross-Strata 

U.S. GEOLOGICAL SURVEY BULLETIN 1670 

Primary Textures in 

Experimentally Formed 

Oolite Cross-Strata 

By CHRISTOPHER J. SCHENK 

U.S. GEOLOGICAL SURVEY BULLETIN 1670 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1987 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225 

library of Congress Cataloging in Publication Data 

Schenk, Christopher j. 

Primary textures in experimentally formed oolite cross-strata. 

(U.S. Geological Survey bulletin ; 1670) 

Bibliography: p. 

Supt. of Docs. No.: I 19.3:1670 

1. Sedimentary structures. 2. Oolite. 3. Sedimentation and 

deposition. I. Title. II. Series. 

QE75.B9 no. 1670 557.3 s 86--600398 

[QE472] [552'.5] 
CONTENTS 

Abstract 1 

Introduction 1 

Methods of study 1 

Observations 2 

Grain-size grading 4 

Sorting 6 

Interparticle porosity 9 

Discussion 9 

Conclusions 10 

References cited 11 

FIGURES 

1. Flow separation and expansion at the brink of a bedform 2 

2. Grain-size distribution of oolitic sand 3 

3. Grain-size increase downslope in a sandflow cross-stratum 5 

4. Inverse grading in a sandflow cross-stratum 6 

5. Sorting downslope in a sandflow cross-stratum 7 

6. Histogram of porosity data 10 

TABLES 

1. Grain-size grading in some sandflow cross-stratification 4 

2. Summary of petrographic observations 8 

Contents Ill 

Primary Textures in Experimentally 

Formed Oolite Cross-Strata 

By Christopher j. Schenk 

Abstract 

Fourteen experiments were conducted to describe the 

primary textures in subaqueous cross-strata formed by sandflows in sediment composed of oolitic sand. Primary textures 

studied were grain-size grading, sorting, and porosity exhibited 

by the experimentally produced cross-strata. Nearly all of the 

cross-strata were formed by the process of sandflow, a response 

to slope instability brought about by continued grainfall deposition on the foreset surface. In the formation of a sandflow crossstratum, a series of small sandflows occurred; most of the preexisting grainfall deposits, as well as some of these smaller sandflows, were redeposited downslope by a large sandflow. The 

processes associated with sandflow produced the textural variations that defined each cross-stratum. 

Each sandflow cross-stratum became coarser grained 

downslope in response to grain rearrangements during the 

causative flows. In many cases, the finer grained oolites were 

removed from the bulk of the flowing mass of oolitic sand and 

accumulated near the plane of shear on the upper part of the 

foreset surface; this process caused some of the sandflow crossstrata on the upper part of the foreset surface to exhibit inverse 

grading. Toward the lower part of the foreset surface, the sandflow cross-strata became coarser grained and were mostly 

ungraded. Such coarser grained cross-strata characteristically 

formed an angular contact with the base of the foreset. Within 

each cross-stratum, because finer grained oolites are removed 

during each sandflow, the sorting improves markedly 

downslope from moderately sorted near the top of the foreset 

to well sorted at the base. The interparticle porosity also increased downslope within each cross-stratum. Porosity on the 

upper half of the foreset ranged from 36 to 45 percent, averaging 40 percent, whereas the porosity on the lower half of the 

foreset ranged from 40 to 47 percent, averaging 45 percent. 

INTRODUCTION 

The purpose of this paper is to describe the primary 

textures in cross-strata produced in oolitic sand on a 1-mlong foreset surface in a laboratory flume. Primary textures exhibited by the experimentally formed cross-strata 

include grain-size grading, sorting, and porosity. The 

development of cross-strata at or near the angle of repose 

results from flow separation and expansion to the lee of 

a bedform (fig. 1). These processes result in decreased 

competency of the flow and cause sediment previously 

transported in suspension and bedload to be deposited 

onto the sloping foreset surface. Most of the coarser 

grained bedload is deposited at the top of the foreset 

slope, a process termed "collective settling" (Jopling, 

1964, 1965). The finer grained bedload and the sediment 

previously in suspension transport are deposited onto the 

foreset surface in such a way that the amount of deposition decreases downslope. Hunter (1977) called this process "grainfall." Continued deposition causes the upper 

part of the foreset surface to attain the angle of initial 

yield, the angle above which slope failure occurs (Allen, 

1970). The resulting sandflow redistributes all or part of 

the unstable grainfall deposit downslope, and high-angle 

sandflow cross-strata are produced. 

Hunter (1977) illustrated that primary textures differ between eolian cross-strata due to grainfall and those 

due to sand flow, which together make up the bulk of 

eolian foreset deposits. He attributed the differences in 

grain-size grading, sorting, and porosity to the processes 

of deposition responsible for the development of each 

type of stratification. Several recent studies of crossstratified, oolitic carbonates have also illustrated textural 

variations in high-angle, foreset deposits (Evans, 1983, 

1984; Halley and Evans, 1983). The experiments described in this paper were undertaken (1) to investigate 

the role depositional processes have in producing textural 

variations in subaqueous cross-strata composed of oolitic 

sand, and (2) to document the primary textures exhibited 

by the cross-strata. 

METHODS OF STUDY 

Fourteen experiments were conducted in a small 

tank (fig. 1) fitted with a 1-m-long foreset slope. A steady 

flow of water transported oolitic sand, introduced to the 

flow by a box situated above the tank, over the brink of 

the foreset surface. Most of the deposition took place on 

Methods of Study 
Collective settling Flow separation 

~~~~~~~~~~~----------of_b_e_d_lo_ad __ ~------~--a_n_d_ex~p_a_ns_io_n ___________ WATERSURFACE-------

Artificial 

foreset slope 

~ ~ ~ IP 0 

.~ Grainfall deposition 

Angular contact with base 

of foreset 

Figure 1. The processes of flow separation and expansion at and beyond the brink of a bedform cause sediment, previously 

transported in suspension and (or) as bedload, to be deposited onto the sloping foreset surface. Continued sedimentation leads 

to slope instability and subsequent failure of the sediment; the result is the formation of a cross-stratum. The experimental set-up 

simulated the processes of flow separation and expansion over the brink of a 1-m-long foreset slope. Oolitic sand was fed into 

the flow from a box above the tank. 

the upper one-fourth of the foreset slope, a situation that 

led to sandflow. After a sufficient thickness (about 15 

em) of oolitic sand was deposited by a succession of sandflows, the experiment was terminated. These experiments 

were not conducted under closely controlled hydraulic 

conditions but were valuable in that the processes of 

grainfall and sandflowage and the development of crossstrata could be observed directly. 

After seven of the experiments, the tank was 

drained and latex peels were prepared in trenches dug in 

the center of the deposit parallel to the dip of the foreset 

slope. The damp sand along the trench was smoothed, 

and cheesecloth was affixed to the sand surface. Liquid 

latex was then painted onto the cheesecloth, and was 

drawn by capillarity into the damp sand behind. After 

several hours, the latex dried and the cheesecloth was 

peeled away from the sand face; as much as 2 em of sand 

from the undisturbed cross-strata adhered to the cloth. 

After allowing the latex to fully cure, oriented 

samples for thin sections were cut from each peel. These 

samples were cut in a vertical orientation parallel to the 

direction of sediment transport; therefore, the strata 

dipped at the angle of repose across each thin section. 

Ten thin sections were prepared from each peel: two each 

from approximately 5, 25, 50, 75, and 100 em downslope. 

All thin sections were impregnated with blue epoxy to 

contrast oolites and intergranular porosity. 

Intergranular porosity was measured on each sample using a point counting technique, and approximately 

650 points were counted on each of the seventy thin sections. Halley (1978) demonstrated that porosity obtained 

by point counting can be subject to several errors, due 

to the effects of grain curvature and so on, but care was 

taken to reduce the possible magnitude of these errors. 

Grain-size grading and sorting were observed directly in 

each thin section, and sorting was compared visually to 

2 Experimentally Formed Oolite Cross-Strata 

diagrams modified from those of Pettijohn and others 

(1972). 

The oolites (about 500 lb) used in the experiments 

were obtained from shoals near Bimini Island, Bahamas, 

by E. D. McKee. The oolites are medium grained, 

moderately well sorted, and fine skewed (using the limits 

of Folk, 1968; fig. 2). The mean size and sorting are 

similar to values given for many oolite deposits, both 

modern and ancient (Carr, 1973). 

OBSERVATIONS 

All of the cross-strata formed by the process of 

sandflow. Most of the sand was deposited onto the upper part of the foreset surface by grainfall, which resulted 

in slope instability and failure and redeposition of the 

grainfall deposits by sandflow. The sandflows, in general, 

did not begin across the entire width of the foreset, but 

rather were localized in one area. Many of the sandflows 

ceased movement before reaching the base of the slope. 

Generally, several sandflows stopped at various levels on 

the slope before one finally reached the base. In the process, the one larger flow that reached the base redeposited 

some sediment from the previous sandflows that did not. 

Each sandflow interfered, to some degree, with the 

stratum it was overriding and caused several grain layers 

to be removed from the upper part of the preexisting 

deposit. These grains were incorporated into the subsequent flow. The original depositional thickness was 

seldom preserved in the cross-strata owing to this ''plowing" process. 

Each sandflow began at the top of the foreset surface as a small slump that quickly degenerated into a 

sandflow, in which all the grains were in motion relative 

to one another. The initial slump and sandflow acted to 
99.0 

Iz 

w 

0 

0:: 

w 

(L 90.0 

~ 

>-" 

I-

::::J 

iii 70.0 

<.( 

I]) 

0 50.0 0:: 

(L 

w 

> 30.0 

F 

<.( 

_J 

:::::> 

~ 

:::::> 10.0 0 

5.0 

2.5 3.0 

GRAIN SIZE, IN ¢ UNITS 

Figure 2. The oolitic sand used in the experiments was 

medium grained (mean= 1.710</>), moderately well sorted 

(u= 0.520¢), and fine skewed (SK= 0.17). The millimeter 

equivalents of phi (</>) units are as follows: 

-gravel-l-sand--1-----silt---l-clayphi (</>) -3 -2 -1 0 4 6 8 9 

mm 8 4 0.5 0.25 0.125 0.062 0.031 0.016 0.008 0.004 0.002 

disperse the grains and resulted in a very loose flow of 

sand. Observations of this process through the glass wall 

of the tank indicated that, shortly after the initiation of 

movement, the relatively coarser grains traveled along the 

sides, front, and top of the snout of the flow; this sorting allowed the finer grained oolites to accumulate along 

the plane of the flow. In the formation of a discrete crossstratum, many small, early sandflows stopped after having moved only a short distance from the top of the 

foreset, and the finer grains were not completely removed 

from these flows. Deposits of several of these small flows 

steepened the slope and allowed a subsequent larger sandflow to incorporate some of these deposits and then to 

proceed to the base of the foreset slope. The mechanism 

that operated to place the finer grained oolites near the 

base of the flows acted quickly, so that most of the finer 

grained oolites remained on the upper one-third of the 

foreset slope. Most of the cross-stratum, therefore, was 

composed of the relatively coarser grains. 

The contacts between most of the individual sandflows were sharp, particularly on the upper part of the 

foreset. Here, the finer grains along the base of each 

cross-stratum provided a distinct textural contrast with 

the coarser oolites of the stratum below. The amount of 

finer grained oolites along the base of each cross-stratum 

decreased significantly about one-third of the way down 

the slope, and therefore, the contacts on the lower part 

of the slope were much less distinct, but still discernible. 

Little grainfall deposition took place on the lower part 

of the foreset surfaces. All of the sandflows on the lower 

part of the foreset were stacked without intercalated 

grainfall deposits. What little grainfall deposition did occur on the lower part of the slope either sifted into the 

interstices of the larger oolites or was plowed up and incorporated into the next sandflow. In all the experiments, 

the sandflows stopped at a fairly constant angle of repose 

of approximately 27 °. The contact between the crossstrata and the base of the foreset was angular in all cases. 

In all the experiments, the sandflows were not continuous, but intermittent, in that there was some time interval between successive sandflows. The frequency of 

sandflow, as well as the duration and amount of movement of each sandflow, was quite irregular. However, 

despite this uneven mode of formation, each set of crossstrata observed in the trenches consisted of a series of 

discrete sandflow cross-strata. Each cross-stratum was 

bounded by more or less sharp contacts representing the 

events in which the larger flows reached the base of the 

slope at the expense of the smaller flows. The larger flows, 

therefore, redeposited two types of existing cross-strata: 

grainfall deposits on the upper part of the foreset and 

many smaller sandflows that had not reached the lower 

half of the slope. Many more smaller flows had taken 

place than were represented in the deposit, as they were 

a partial source of sand for the larger flows. The larger 

sandflows were always more laterally extensive, involving nearly the entire area of the foreset, whereas the 

smaller flows were much more localized and lobate in 

shape. 

The regularity of the experimentally produced 

cross-strata resulted from the deposits of these larger 

sandflows, the primary sources for which were sand 

deposits already on the foreset slope. A critical volume 

of sand was necessary before a sandflow could reach the 

base of the slope. This volume of sand was built up by 

the numerous small sandflows that did not reach the base 

of the slope. The regularity of the cross-strata had no relation to the movement of smaller scale bedforms across 

the topset surface-as there was none-or to sediment 

"pulses" either by volume or by grain size over the brink. 

Smith's (1972) field observations of bedforms in the 

Observations 3 
Platte River led him to conclude that the movement of 

ripples and ripple troughs across the stoss slope and over 

the brink of fluvial sand waves was responsible for the 

textural variations in the foreset deposits, because the 

troughs were coarser grained than the ripples themselves. 

Jopling (1964) concluded on the basis of flume experiments that processes upstream of the brink of the 

foreset acted to segregate grain sizes and thus constituted 

a first-order control on the textural variations in crossstrata. Deposition of alternating coarser and finer grained 

sand would produce repetitive textural variations. 

However, these two interpretations cannot be generally 

applied, as similar textural variations can arise by sandflow alone, as demonstrated here. 

Grain-Size Grading 

The most obvious visual grain-size grading in each 

cross-stratum is in a downslope direction (fig. 3). This 

Table 1. Grain-size grading in some sandflow cross-strata 

[Leaders (---) indicate no data] 

Cross-strata 

environment 

Subaqueous 

Do 

Do ----------

Do 

Experimental 

subaqueous 

Do ----------

Theoretical 

sandflows 

Experimental 

subaqueous 

Subaqueous 

Do 

Do 

Do 

Do 

Experimental 

subaqueous 

& subaerial 

Experimental 

subaerial 

Do 

Subaerial 

Do 

Do 

Do 

Observed grading 

Coarsening downslope 

do 

do 

Coarsening downslope, 

normal grading 

Coarsening downslope 

do 

Coarsening downslope, 

inverse grading 

Coarsening downslope 

do 

do 

do 

do 

do 

Inverse grading 

Coarsening downslope, 

subtle inverse grading 

Coarsening downslope, 

weak inverse grading 

Coarsening downslope, 

inverse grading 

Coarsening downslope, 

indistinct inverse grading 

Coarsening downslope, 

inverse to normal grading 

Coarsening downslope, 

inverse grading 

4 Experimentally Formed Oolite Cross-Strata 

grading was observed in each cross-stratum, both where 

the deposits lay against the glass wall of the tank and 

where they were cut by the trenches, which were located 

in the center of the tank. Previous studies of cross-strata 

have illustrated this grading in many different types of 

bedforms from diverse environments, as well as in several 

studies of experimentally formed cross-strata (table 1). 

The observations made during this study on the 

events leading to this downslope grading generally follow 

the descriptions outlined in previous experimental studies 

by Allen (1965) and Jopling (1964; 1965). After the initiation of movement, the finer grains somehow accumulated near the base of the sandflow on the upper 

part of the foreset, leaving the coarser grains to make 

up the bulk of the flow that reached the base of the slope. 

The nature of the mechanism that operated to concentrate the finer grains near the base of the flow was interpreted to be dispersive pressure, as formulated by 

Bagnold (1954). Bagnold theorized that the coarser grains 

would be forced to the upper, free surface of the flow 

Type of sand 

Modern oolitic 

Jurassic oolitic 

Pleistocene oolitic 

Mississippian oolitic 

Siliciclastic 

___ do __ _ 

Siliciclastic 

Cretaceous siliciclastic 

Modern siliciclastic 

Pennsylvanian siliciclastic 

Modern siliciclastic ___ do __ _ 

Siliciclastic 

___ do __ _ 

___ do __ _ 

___ do __ _ 

___ do __ _ 

___ do __ _ 

___ do __ _ 

Reference 

Imbrie and Buchanan (1965). 

Klein (1965). 

Evans (1983). 

Handford (1978). 

Jopling (1964, 1965). 

Allen (1965). 

Bagnold (1954). 

Allen (1968). 

Allen and Narayan 

(1964). 

Collinson ( 1970). 

Visher (1965). 

Ore (1964). 

Smith (1972). 

Sallenger (1979). 

Fryberger and Schenk 

(1981). 

Schenk (1983). 

Inman and others (1966). 

Hunter (1977). 

Ahlbrandt and Fryberger 

(1982). 

Kocurek and Dott (1981). 
0 

C"' 

<ll 

~ 

::;'! 

~ 

o· 

::s 

<ll 

VI 

Figure 3. Photomicrographs showing that the median grain size of the oolites increased downslope in each cross-stratum. This is the most pronounced textural characteristic 

of sandflow cross-stratification . A, Sample E2, 6 em downslope; 8, sample E3, 24 em downslope; C, sample E5, 49 em downslope; 0, sample E10, 99 em downslope. 

All four photomicrographs were taken at the same scale (bar= 0.2 mm) to facilitate comparisons. 
by grain collisions, as these forces would be greatest on 

the larger grains. Concomitantly, the finer grains would 

be forced to the base of the flow. Middleton (1970) and 

Middleton and Southard ( 1978) suggested that this 

phenomenon may be kinetic rather than dynamic, in that 

the finer grains would simply sift down through the interstices of the larger grains during dilation of the sand, 

shortly after initial movement. However, Sallenger (1979) 

concluded from a series of experiments on grading in 

sandflow cross-strata that dispersive pressure may indeed 

produce this grain-size segregation. 

One of the characteristics of this grain-size segregation, at least for the oolitic sand used in this study, is that 

each cross-stratum from about midslope to the base was 

composed almost entirely of the coarser oolites. The finer 

grained oolites, for the most part, were concentrated in 

the lower part of each cross-stratum on the upper part 

of the foreset slope. Some of the cross-strata were inversely graded on the upper part of the foreset slope because 

of the presence of these finer grained oolites. This 

characteristic diminished downslope. The coarser grained 

parts of each cross-stratum (downslope) generally were 

not visibly graded in the direction normal to the plane 

of the foreset (fig. 4). In thin section, a few of the coarser 

grained foresets exhibited a slight normal grading, in 

which the grain size diminished slightly, in a direction upwards from and normal to the plane of the foreset. 

Ahlbrandt and Fryberger (1982) reported normal grading 

in some subaerial sandflow cross-strata (table 1). 

Horizontal exposures were made at several levels 

downslope in two of the experimental deposits. In these 

exposures the thickness of each cross-stratum was exaggerated, and the vertical grain size grading (normal to the 

foreset slope) in a cross-stratum was accentuated. These 

exposures demonstrated that some of the cross-strata were 

inversely graded only on the upper part of the foreset 

slope, whereas the cross-strata on the lower part of the 

slope were coarser grained and largely ungraded. Differences in grain packing probably resulted in the slight 

textural variations that define each cross-stratum on the 

lower part of the foreset. 

Sorting 

Visual estimates indicated that sorting improved 

downslope within each cross-stratum (fig. 5), as a result 

of fewer finer grained oolites in this direction (table 2). 

The improved sorting can be attributed to the mechanism 

Figure 4. Photomicrograph illustrating inverse grading exhibited by a cross-stratum on the upper part of 

the foreset slope. The arrows mark the boundary between the finer grained lower part of a cross-stratum 

A, and the coarser grained upper part of underlying stratum B. The inverse grading is "dirty" in that some 

coarser grained oolites were present among the finer grain oolites. Sample 04, 25 em downslope. Bar scale= 1 

mm. 

6 Experimentally Formed Oolite Cross-Strata 
0 

C"' 

Ill 

n> 

~ 

~ 

s· 

::::s 

Ill 

'I 

Figure 5. Photomicrographs showing that sandflow caused the sorting to improve downslope within each sandflow cross-stratum. The finer grained oolites moved toward 

the base of each flow during movement and were deposited on the foreset surface. The sand that reached the base of the slope was relatively free of finer grained oolites 

(see also fig. 4A-0). A, Sample C2, 3 em downslope; 8, sample C4, 25 em downslope, C, sample C6, 50 em downslope, 0, sample C10, 100 em downslope. All four 

photomicrographs were taken at the same scale (bar= 0.2 mm) to facilitate comparisons. 
Table 2. Summary of petrographic observations 

[Leaders ( ---) indicate no grading; M, moderately sorted; MW, moderately well sorted; W, well sorted] 

Sample Distance Sorting Grain size Interparticle 

number downslope (visual estimate) grading porosity 

(em) (percent) 

AI 2 M 40 

A2 4 M 41 

A3 23 M inverse 40 

A4 24 M 38 

AS 49 M inverse 40 

A6 50 M to MW inverse 42 

A7 72 MW 44 

AS 73 MW 43 

A9 97 MW 47 

AlO 98 MWto W 47 

Bl II M inverse 38 

B2 I2 M 38 

B3 24 M 39 

B4 25 M 36 

B5 52 MW 40 

B6 53 MW 42 

B7 77 MWto W 44 

B8 78 MWto W 42 

B9 98 w 46 

BIO 99 w 47 

CI 2 M inverse 37 

C2 3 M 36 

C3 24 M 37 

C4 25 M toMW 40 

cs 49 MW 40 

C6 50 MW 40 

C7 74 MWtoW 45 

C8 75 w 45 

C9 99 w 45 

CIO IOO w 45 

01 4 M inverse 43 

02 IO M inverse 4I 

03 24 M 4I 

04 25 M 45 

05 49 MW 45 

06 50 M 44 

07 74 MW 44 

08 75 MW normal 44 

09 96 w 45 

010 99 w 46 

EI 5 M 40 

E2 6 M 40 

E3 24 M 40 

E4 25 M inverse 43 

E5 49 MW 43 

E6 50 M to MW 45 

E7 74 MWtoW 45 

E8 75 MW toW 46 

E9 98 w 46 

EIO 99 w 45 

Fl 3 M inverse 39 

F2 4 M inverse 40 

F3 24 M 42 

F4 25 M inverse 4I 

8 Experimentally Formed Oolite Cross-Strata 
Table 2. Continued 

Sample Distance Sorting 

number downslope (visual estimate) 

Grain size 

grading 

Interparticle 

porosity 

(em) (percent) 

F5 49 M 

F6 50 MtoMW 

40 

42 

45 

45 

45 

47 

F7 74 MtoMW 

F8 75 MW normal 

F9 98 MW 

FlO 99 w 

Gl 3 M 

G2 4 M 

G3 25 M 

G4 27 M 

G5 49 MW 

G6 50 MW 

G7 74 MW 

G8 75 MW 

G9 97 w 

GlO 98 MWtoW 

36 

39 

39 

43 

43 

44 

44 

43 

44 

46 

that operated to remove the finer grains from the bulk 

of each sandflow during movement. There is little information from the literature with which to compare this 

type of sorting within a single sandflow cross-stratum. 

Ore (1964) examined some foreset cross-strata in sand 

waves of the South Platte River and found the opposite 

relationship; that is, sorting decreased downslope within 

a cross-stratum. 

Interparticle Porosity 

Measurements of porosity on seventy samples of 

impregnated oolitic cross-strata averaged 42.5 percent 

with a standard deviation of 3 percent (fig. 6). The porosity increased in a downslope direction within each crossstratum. Interparticle porosity on the upper half of the 

foreset was as low as 36 percent, as high as 45 percent, 

and averaged 40 percent, whereas on the lower half of 

the foreset, the porosity was as low as 40 percent, as high 

as 47 percent, and averaged 45 percent. The increase in 

porosity downslope coincided with an improvement in 

the sorting in that direction. Many studies have demonstrated that as sorting improves, the porosity increases 

(Fraser, 1935; Rogers and Head, 1961; Beard and Weyl, 

1973). Graton and Fraser (1935) and more recent workers 

have demonstrated that packing also influences porosity. Loosely packed sand exhibits porosities as high as 50 

percent, whereas tighter packed sand can exhibit 

porosities as low as 25 percent. According to these extremes, the range of porosities measured during this study 

are on the high end; these results reflect the loose packing of sand deposited by the process of sand flow. 

Measurements of porosity in thin section by point 

counting can be subject to several errors, all of which act 

to slightly underestimate the true inter granular porosity. 

The use of blue epoxy and ultrathin thin sections probably reduced the magnitude of these possible errors. In 

addition, only the interparticle porosity was evaluated in 

thin section; intraparticle porosity, if present, was not included because it simply cannot be resolved in thin section. Therefore, the porosity data in table 2 consists of 

interparticle porosity only. 

DISCUSSION 

These experiments have demonstrated that each 

oolitic sandflow cross-stratum is composed mostly of the 

coarser grained oolites, which are arranged in angle-ofrepose cross-strata that terminate abruptly at the base of 

the foreset slope. The finer grained oolites are present 

mainly in the lower part of the cross-strata at the top of 

the foreset. The inverse grading produced the textural 

contrasts that outline the cross-strata. The textural contrasts diminish rapidly downslope, as the content of finer 

grains decreases, such that coarser, ungraded' sandflow 

cross-strata are in depositional contact with each other. 

These textural variations were the result of sandflow and 

processes that are associated with sandflow because there 

were no grainfall deposits intercalated with the crossstrata. 

Grain size of grainfall cross-strata characteristically 

decreases downslope, unlike the texture of sandflow 

cross-strata (Hunter, 1977; Schenk, 1983). The juxtaposition of these two types of cross-strata would result in a 

marked textural contrast that could be present along the 

bounding surface all the way down to the base of the 

slope. Evans (1983) described "couplets" of coarser and 

finer grained oolite laminae in foreset deposits of the 

Pleistocene Miami Limestone. These couplets extended, 

in most cases, to the base of the foreset slope. The results 

of these experiments suggest that these couplets were 

probably not formed by sandflow alone, but are the result 

of grainfall deposition and periodic sand flow. The 

resulting foreset deposit is characterized by finer grained 

grainfall cross-strata intercalated with coarser grained 

sandflow cross-strata. Two other attributes of grainfall 

Discussion 9 
40------------------------

N 

30 

>- u 

z 

~ 20 

0 

w 

0:: 

LJ.. 

10 

a~--_.~~~~~~~~--~ 

30 40 50 

PRIMARY POROSITY, 

IN PERCENT 

Figure 6. Interparticle porosity of the oolitic crossstrata ranged from a low of 36 percent to a high of 47 

percent, with an average of 42.5 percent and a 3 percent standard deviation. Porosity generally increased 

J downslope within each cross-stratum. Most of the 

higher porosity values were recorded from strata on the 

lower half of the foreset. 

cross-strata are that sets thin downslope and in many 

cases form tangential contacts with the base of the foreset 

slope. Tangential contacts, possibly formed by grainfall 

deposits, are common in the Miami Limestone (Evans, 

1983). 

The regular alternation of the coarser and finer 

grained foreset deposits in the Miami Limestone is one 

of its most striking attributes. The experiments reported 

here suggested that this regularity can be produced simply 

by continuous grainfall deposition, punctuated by 

periodic sandflow; therefore, there is no need to invoke 

special hydrodynamic conditions, such as steady or 

uniform flow. The sandflows in the experiments were 

quite irregular in size and temporal spacing, but the sets 

of cross-strata were of similar dimensions. This 

phenomena occurred because only the largest sandflows 

reached the base of the foreset slope. In the process, these 

10 Experimentally Formed Oolite Cross-Strata 

larger sandflows redeposited the oolites in the grainfall 

deposits and the smaller sandflow deposits. It appeared 

that once a critical volume of sand was available, a large 

sandflow would occur and would reach the base of the 

slope; the same volume would be needed to produce the 

next cross-stratum. The fixed volume produced the 

regularity of the foreset cross-strata. 

Based on the published photographs of the Miami 

Limestone, in contrast, it appeared that grainfall deposits 

were preserved between discrete sandflows that reached 

the base of the slope. There are several possible reasons 

for this basic difference. First, the experimental 

hydrodynamic regime may have been quite different from 

the regime that formed the cross-strata in the Miami 

Limestone, resulting in a higher rate of grainfall deposition. Second, the bedforms responsible for the sets of 

oolitic cross-strata may have been smaller than the 1-mlong foreset surface used in the experiments; grainfall 

could then have been deposited farther down the foreset 

slope and preserved between the successive sandflows. 

Finally, the Pleistocene oolites that were deposited to 

form the Miami Limestone may have had a predepositional range of grain sizes that was significantly wider 

than the oolites used in these experiments. Thus, there 

could have been a greater proportion of finer grained 

oolites available for grainfall deposition during formation of the Miami Limestone. 

The sets of cross-stratified oolitic sand in the Miami 

Limestone are variable in thickness, but most of the sets 

are about 30 em thick (Evans, 1984). The degree of preservation of a bedform during migration is dependent upon 

the angle of bedform climb relative to the stoss-side slope, 

among other factors (Rubin and Hunter, 1982). The 

smaller the climb angle relative to the stoss-side angle, 

the smaller the amount of bedform preserved. The results 

of the experiments reported here indicated that-because 

of the increase in grain size in each sandflow crossstratum-as less of the upper part of each bedform is 

preserved, the remaining cross-strata are coarser grained 

and better sorted and exhibit higher interparticle 

porosities. 

CONCLUSIONS 

These experiments have demonstrated that sandflow influenced the primary textural characteristics of 

oolitic foreset cross-strata. Some sandflow cross-strata 

exhibited inverse grading on the upper part of the foreset, but became largely ungraded on the lower part. The 

accumulation of finer grained oolites on the upper 

part of the foreset led to an improvement in sorting 

downslope. Along with sorting, the interparticle porosity also increased downslope within each sandflow 

cross-stratum. 
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