pyrite mineralization in microbial mats from the mid proterozoic newland formation belt supergroup montana u s a abstract the mid proterozoic newland formation a shale dominated unit of the belt supergroup was deposited in an eastern extension of the belt basin the helena embayment a variety of different shale facies types can be distinguished of particular interest for this study is a shale facies that has been interpreted to be the result of microbial mat growth resulting in carbonaceous shale beds interrupted by storm deposition causing deposition of graded silt mud couplets alternation of carbonaceous beds with silt mud couplets gives these shales a characteristic striped appearance along the basin margins a pyrite rich sub facies of these striped shales is found locally consisting of laminated pyrite beds that alternate with non pyritic silt mud couplets laminated pyrite beds in pyritic striped shales are interpreted as mineralized microbial mats because of wavy crinkly internal laminae and because of the direct association with unmineralized striped shales that contain microbial mat deposits excess iron in pyritic shale horizons was probably supplied by terrestrial runoff in colloidal form iron hydroxides introduced by rivers into basin marginal lagoons flocculated and were then incorporated into microbial mats and reduced to pyrite upon burial

impressions of algal mats from the middle proterozoic belt supergroup northwestern montana u s a irregular low relief bedding plane markings which probably represent impressions of thin wrinkled algal mats have recently been detected in mudstones from the 1100 m y old snowslip formation belt supergroup glacier national park montana algal mat impressions are rarely reported from ancient strata yet algal mats are common in certain modern environments originating primarily in moist settings and in relatively quiescent shallow water settings such structures may be of some value in interpreting ancient depositional environments

environmental interpretation and a sequence stratigraphic framework for the terminal proterozoic ediacara member within the rawnsley quartzite south australia abstract fossils of the ediacara biota are confined to preservational windows in the ediacara member of the rawnsley quartzite pound subgroup from the flinders ranges south australia the base of the ediacara member is a type 1 sequence boundary incised into the partly lithified chace quartzite member of the rawnsley quartzite the ediacara member and the upper half of the rawnsley quartzite comprise the rawnsley depositional sequence bounded above by the early cambrian uratanna depositional sequence the rawnsley sequence developed over an erosional surface cut into the chace quartzite member with some 250 m of relief southeasterly directed palaeovalleys are filled with a sequence of massive sandstone and laminated siltstone passing up into well bedded sandstone at lowstand local channels were filled by massive sand the highstand systems tract consists of several parasequence sets of laminated siltstone and channelized amalgamated massive sandstone overlain by interbedded siltstone and sandstone grading up into well bedded clean sandstone impressions of soft bodied ediacaran organisms are preserved above the valley fill facies on sandstone partings within upward shoaling delta front environments between storm and fairweather wavebase the ediacara member is in places conformably overlain by up to 500 m of sandstone of the same lithofacies as those underlying the member distinctive microbial mat bound sand laminae described as petee structures are characteristic of the rawnsley quartzite above and below the ediacara member during deposition of the ediacara member colonization of the deeper substrates by microbial mat communities enabled preservation of the ediacara biota the erosional surface at the base of the ediacara member is one of at least four incision events recorded in the terminal proterozoic and early cambrian succession of the adelaide geosyncline the similarity of incision filling lithofacies of the rawnsley and uratanna sequences has in condensed sections led to confusion between the depositional sequences characteristic trace and body fossil assemblages provide the only clear distinction between depositional cycles spanning the proterozoic cambrian boundary

stromatolites the challenge of a term in space and time abstract several definitions of stromatolites are discussed and kalkowsky s most important statements about stromatolites are translated from german to english a new definition for stromatolites is proposed namely stromatolites are laminated rocks the origin of which can clearly be related to the activity of microbial communities which by their morphology physiology and arrangement in space and time interact with the physical and chemical environment to produce a laminated pattern which is retained in the final rock structure unconsolidated laminated systems clearly related to the activity of microbial communities and often called recent stromatolites or living stromatolites are defined as potential stromatolites the main microbial activities which are important in the formation of potential stromatolites and stromatolites are described and examples of stromatolites of various types including iron stromatolites are also described

heat flow in the ozark plateau arkansas and missouri relationship to groundwater flow abstract heat flow values were calculated from direct measurements of temperature and thermal conductivity at thirteen sites in the arkansas missouri ozark plateau region these thirteen values are augmented by 101 estimates of heat flow based on thermal conductivity measurements and temperature gradients extrapolated from bottom hole temperatures the regional heat flow profile ranges from 9 mw m 2 to over 80 mw m 2 but at least two distinct thermal regimes have been identified seven new heat flow determinations are combined with three previously published values for the st francois mountains sfm a precambrian exposure of granitic and rhyolitic basement rocks average 47 mw m 2 radioactive heat production of 76 samples of the exposed rocks in the sfm averages 2 4 w m 2 and a typical continental basement contribution of 14 mw m 2 is implied conversely the sedimentary rock sequence of the plateau is characterized by an anomalously low heat flow averaging approximately 27 mw m 2 groundwater transmissivity values that are based on data from 153 wells in deep regional aquifers demonstrate an inverse relationship to the observed heat flow patterns the areas of high transmissivity that correspond to areas of low total heat flux suggest that the non conservative vertical heat flow within the ozark sedimentary sequence can be attributed to the effects of groundwater flow

electrical conductivity structure of southeastern north america implications for lithospheric architecture and appalachian topographic rejuvenation abstract we present the first three dimensional view of the lithospheric electrical conductivity structure beneath southeastern north america by inverting earthscope long period magnetotelluric mt data we obtain an electrical conductivity image that provides new insights into both the architecture of the appalachian orogen and the cryptic post rifting geodynamic history of the southeastern united states our inverse solutions reveal several elongate electrically conductive features that we interpret as major terrane sutures within the appalachian orogen most significantly we resolve a highly electrically resistive layer that extends to mantle depths beneath the modern piedmont and coastal plain physiographic provinces as high resistivity values in mantle minerals require cold mantle temperatures the mt data indicate that the sub piedmont thermal lithosphere must extend to greater than 200 km depth this firm bound conflicts with conclusions from seismic results the boundary between the anomalously thick resistive sub piedmont lithosphere and the relatively thin moderately conductive sub appalachian lithosphere corresponds within resolution to the modern appalachian topographic escarpment this newly recognized contrast in lithospheric properties likely has important implications for appalachian topographic rejuvenation

major influences on the evolution of the 2 67 2 1 ga transvaal basin kaapvaal craton abstract the transvaal is one of three structural basins of the transvaal supergroup preserved on the kaapvaal craton the evolution of the neoarchaean palaeoproterozoic transvaal basin is ascribed predominantly to magmatism palaeoclimate and eustasy with plate tectonics playing a sporadic role the supergroup comprises basal protobasinal rocks followed by the black reef formation chuniespoort group and uppermost pretoria group immature siliciclastic and bimodal volcanic rocks of the protobasinal unit reflect a wide zone of rifting related to the c 2 7 ga ventersdorp supergroup mantle plume individual protobasinal successions were laid down in separate fault bounded basins controlled at least partially by greenstone belt orientations in the kaapvaal basement post magmatic post rifting thermal subsidence accommodated black reef fluvial sheet sandstones and the subsequent thick carbonate bif epeiric platform succession of the chuniespoort group subordinate mechanical subsidence accompanied this long lived thermal relaxation intense weathering due to neoarchaean atmosphere composition greatly reduced clastic sedimentation and the greenhouse palaeoclimate further encouraged carbonate sedimentation globally enhanced sea levels due to enhanced mid ocean ridge growth consequent upon either global magmatic events or supercontinent break up also played a pivotal role in chuniespoort epeiric basin evolution pretoria group sedimentation is ascribed to two cycles of rifting and subsequent thermal subsidence the first cycle appears to reflect plate tectonically induced rifting with an epeiric sea drowning the rift basin despite reduced sea levels due to the first major global glaciation the second pretoria cycle is most likely related to a major continental flood basalt event with thermal subsidence allowing a second and probably larger epeiric sea to advance onto the northern kaapvaal craton evaluation of the inferred relatively minor importance of plate tectonics in the evolution of the essentially intracratonic transvaal basin depends to a large degree on the age assigned to the limpopo orogeny during which the zimbabwe and kaapvaal cratons collided recent evidence for a c 2 0 ga collision supports the concept that catastrophic global magmatic events gradually gave way to plate tectonics during transvaal basin evolution

could bacteria have formed the precambrian banded iron formations banded iron formations bifs are prominent sedimentary deposits of the precambrian but despite a century of endeavor the mechanisms of their deposition are still unresolved interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy on the basis of chemical analyses from bif units of the 2 5 ga hamersley group western australia we show here that even during periods of maximum iron precipitation most if not all of the iron in bifs could be precipitated by iron oxidizing bacteria in cell densities considerably less than those found in modern fe rich aqueous environments those ancient microorganisms would also have been easily supported by the concentrations of nutrients p and trace metals v mn co zn and mo found within the same iron rich bands these calculations highlight the potential importance of early microbial activity on ancient metal cycling

microbial mat control on siliciclastic precambrian sequence stratigraphic architecture examples from india abstract an attempt has been made to understand within a cause and effect framework the sequence building pattern in proterozoic time that witnessed non uniformiteranean microbial mat growth and epeiric sea development marine or marginal marine successions in three neoproterozoic formations the sonia sandstone in western india and the sirbu shale and upper bhander sandstone in central india have been examined here for this purpose the strikingly common feature of all three formations is vertical stacking of highstand systems tracts without intervention of any significant transgressive deposits in one instance only the transgressive systems tract is represented by a thin granular transgressive lag and in all other cases the evidence of transgression is simply marked by the presence of marine flooding surfaces the absence of transgressive strata relates to the low sea floor gradients which facilitated rapid transgressions combined with a generally low sediment supply aggradation under normal regressive highstand conditions in spite of the low sediment supply was promoted by the prolific growth of microbial mats which reduced the effects of wave and current reworking by organic binding of clastic particles

microbial sand chips a non actualistic sedimentary structure abstract microbial mats show a marked decline with the transition from the precambrian to the phanerozoic most likely due to the advent of effective benthic grazing organisms hitherto most aquatic sediments up to sand grade were commonly coagulated by organic material allowing reworking to form intraclasts new term microbial sand chips presence of a microbial mat is indicated 1 by refractory remains of organic matter in the laminated parent sediment 2 by the concentration of early diagenetic pyrite associated with sandy intraclasts in quartzites indicating a concentration of organic matter in the clast and 3 by plastic deformation of the clasts during the erosive event as bioturbation spread into most aquatic depositional environments during the phanerozoic the formation of microbial sand chips became restricted to habitats where bioturbation and grazing are limited by environmental stress and where the preservation potential for microbial sand chips is very low therefore the widespread formation of microbial sand chips in precambrian sandy subaqueous sediments is non actualistic

wrinkle structures microbially mediated sedimentary structures common in subtidal siliciclastic settings at the proterozoic phanerozoic transition sedimentary structures produced by microbial activity are well known from ancient carbonate facies but little is known about equivalent microbial structures in ancient siliciclastic facies wrinkle marks and kinneyia ripples are closely related sedimentary structures that are commonly found in ancient siliciclastics and they may represent microbial activity to evaluate this possibility these structures were studied in vendian cambrian strata of north america and compared to structures formed by modern microbial mat communities in redfish bay texas striking similarities in sedimentologic petrographic and morphologic characteristics of modern and ancient occurrences suggest that the enigmatic but widespread ancient structures could have been formed by microbial processes the relative abundance of these structures in siliciclastic facies of this interval further suggests that prior to the onset of relatively intense bioturbation in the ordovician microbial mats could have played a significant role in the evolution and diversification of early life in a broad variety of sea floor environments

the ongeluk basaltic andesite formation in griqualand west south africa submarine alteration in a 2222 ma proterozoic sea abstract the ongeluk lavas form part of the palaeoproterozoic transvaal griqualand west supracrustal sequence of the archaean kaapvaal craton of south africa they form a thick shallow marine volcanic sequence of pillow lava massive flows and hyaloclastite which together with their comagmatic subaerial hekpoort correlate once covered most of the craton in this study the magmatic composition and alteration features of ongeluk samples were distinguished using geochemistry and radiogenic isotopes the ongeluk hekpoort magma was a basaltic andesite derived from anomalous kaapvaal craton lithosphere with a long history of high u pb high rb sr and chur like sm nd ratios little evidence for magmatic fractionation was found although contamination of the lower lavas with cr rich material is evident in both hekpoort and ongeluk sequences the alteration of ongeluk lavas is characterised using diagrams which compare pillow core rim pairs hyaloclastite and massive lava samples with the estimated original magma composition alteration is ascribed to a shallow 87 sr 86 sr ratios decreased from magmatic 0 7071 towards seawater 0 7025 during submarine alteration the rb sr system also reveals a much later 1200 ma phase of ca and rb mobility in a few samples in contrast the submarine alteration affected pb isotope ratios so little that a 2222 13 ma isochron age is regarded as a refinement of earlier dates for the ongeluk hekpoort extrusion the ongeluk hyaloclastites acted as a sink for k and rb in the overlying sea while the mn and ca entering the sea could have contributed to the overlying kalahari manganese deposits

sedimentology of hot spring systems hot springs and geysers hold a special fascination for many people to the casual visitor and tourist they provide a powerful visual display of nature at work if their temperature is tolerable taking the waters can provide relaxation comfort and alleged health benefits to the scientist however terrestrial hydrothermal systems are of major importance in many disciplines they serve as models for understanding the processes in the shallowest part of the geothermal systems that produce epithermal mineral deposits they are commonly the surface manifestation of large subsurface geothermal systems that may be tapped for geothermal energy their fascinating ecosystems are dominated by diverse arrays of microorganisms many of which thrive in waters of extremely high temperatures and compositions that are toxic to all other organisms although the life of a geothermal system may be geologically brief typically lasting only a few hundred to a few hundred thousand years many hot springs leave a tangible record of former activity in the deposits that were precipitated in and around their vents these hot spring deposits which are composed mainly of calcium carbonate travertine and silica siliceous sinter are the topic of this special issue hot spring deposits have been studied seriously for more than a century the pioneer of their detailed study was walter harvey weed who in the 1880s was one of the first to examine the deposits of siliceous sinter and travertine at yellowstone national park in wyoming u s a more than a century ago weed 1889 a 1889 b 1889 c recognized that the abundant microorganisms that inhabit spring environments may play a role in the formation of travertine and sinter the subsurface geothermal systems that feed hot springs were widely studied with a view to understanding and predicting epithermal mineralization interest in surface hot spring deposits however

candidate sites for future hot dry rock development in the united states generalized geologic and other data are tabulated for 24 potential hot dry rock hdr sites in the contiguous united states the data show that hdr resources occur in many geologic and tectonic settings potential reservoir rocks at each prospect are described and each system is cateogrized accoridng to inferred heat sources the fenton hill area in new mexico is discussed in detail because this region may be considered ideal for hdr development three other prospectively valuable localities are described the geysers clear lake region in california the roosevelt hot springs area in utah and the white mountains region in new hampshire these areas are singled out to illustrate the roles of significantly different geology and geophysics reservoir rocks and reservoir heat contents in possible hdr developments

terminal proterozoic mid shelf benthic microbial mats in the centralian superbasin and their environmental significance abstract a combined sedimentological and biogeochemical study has been conducted on several terminal proterozoic mid shelf microbial mat facies from the centralian super basin isotopic and organic geochemical analysis of the bitumen and kerogen indicated that two sources of organic matter from planktonic and benthic microbial mat populations contributed to the sediment the planktonic source provided a suite of n alkanes with c20 whereas the benthic source contributed an overlay of n alkanes c20 with a strong even preference together with mid chain methyl alkanes kerogen and biomarkers derived from the microbial mat were found to be depleted in 13c relative to planktonic material pyrite in the micorbial mats was also found to be depleted in 34s compared to surrounding facies the combination of these observations suggested that the mats may have been at least partly composed of sulfide oxidising bacteria these organisms have specific environmental tolerances that set limits on palaeo environment their requirement for oxygen indicates that the water column above the mid shelf could not have been anoxic accordingly from the results and age determinations reported here it would appear that mid shelf environments of the centralian superbasin of australia were seeing significant levels of oxygen through the ediacarian

joint hypocenter velocity inversion for the eastern tennessee seismic zone a joint hypocenter velocity inversion for the eastern tennessee seismic zone etsz has resolved velocity features in basement rock below detached appalachian thrust sheets p and s wave arrival times from 492 earthquakes have been inverted for one 1 d and three dimensional 3 d velocity models to midcrustal depths the 3 d p and s wave velocity solutions are computed independly and are very similar in relation to the 1 d model velocity anomalies range from 8 to 16 in the first layer upper 5 km and between 7 in deeper layers prominent velocity anomalies parallel the seismic zone and are consistent from layer to layer the most persistent anomaly is a low velocity region that borders the seismic zone to the northwest and is flanked on either side by regions of anomalously high velocity the new york alabama ny al magnetic lineament coincides with or lies close to the southeast boundary of the prominent velocity low in both the p and s wave velocity images the spatial coincidence between velocity gravity and magnetic gradients suggests that major discontinuities are present in the basement relocation in the 3 d velocity model reduced the number of very deep earthquakes below 20 km and further accentuated differences in seismogenic properties on either side of the ny al lineament after relocation most earthquakes occur in a vertically bounded region roughly 30 km wide extending from 4 to 22 km in depth most earthquakes occur in regions characterized by either average velocity or small velocity anomalies

possible indicators of microbial mat deposits in shales and sandstones examples from the mid proterozoic belt supergroup montana u s a abstract it has been suspected for some time that microbial mats probably colonized sediment surfaces in many terrigenous clastic sedimentary environments during the proterozoic however domination of mat morphology by depositional processes post depositional compaction and poor potential for cellular preservation of mat building organisms make their positive identification a formidable challenge within terrigenous clastics of the mid proterozoic belt supergroup a variety of sedimentary structures and textural features have been observed that can be interpreted as the result of microbial colonization of sediment surfaces among these are a domal buildups resembling stromatolites in carbonates b cohesive behaviour of laminae during soft sediment deformation erosion and transport c wavy crinkly character of laminae d bed surfaces with pustular wrinkled appearance e rippled patches on otherwise smooth surfaces f laminae with mica enrichment and or randomly oriented micas g irregular curved wrinkled impressions on bedding planes h uparched laminae near mud cracks resembling growth ridges of polygonal stromatolites and i lamina specific distribution of certain early diagenetic minerals dolomite ferroan carbonates pyrite although in none of the described examples can it irrefutably be proven that they are microbial mat deposits the observed features are consistent with such an interpretation and should be considered indicators of possible microbial mat presence in other proterozoic sequences

ripple marks in quartz arenites of the hurwitz group northwest territories canada evidence for sedimentation in a vast early proterozoic shallow fresh water lake abstract supermature quartz arenites of the whiterock member deposited in the early proterozoic intracratonic hurwitz basin have a maximum thickness of 400 m and a minimum areal extent of 100 000 km2 the unit is unusual in that it is virtually devoid of other rock types and displays sedimentary structures that consist almost entirely of short spaced ripple marks and parallel stratification both formed by oscillatory flow the ripples have spacings that are consistently small average

heat flow in the piedmont and atlantic coastal plain of the southeastern united states seventy four new heat flow values have been determined at sites from new jersey to georgia mostly in the piedmont and atlantic coastal plain provinces relatively high heat flows are characteristic of synmetamorphic and postmetamorphic granites exposed in the piedmont and occurring in the basement beneath the sediments of the atlantic coastal plain the piedmont heat flow and heat generation values determined in granites metagranites and one slate belt site lie in a belt approximately parallel to major structural trends in the appalachians these values fall on or near the regression line q mw m 2 29 8 n 1 5 mw m 2 7 8 km n 0 475a w m 3 r 0 97477 it is proposed that the occurrence of a thrust fault that truncates granites at a depth d of 7 8 km is directly responsible for this linear relation in the eastern piedmont basement core from beneath the sediments of the atlantic coastal plain was recovered at ten sites results from five of these sites plus one site in the baltimore gabbro define a second linear relation with a slope d 8 0 q 0 380 km r 0 99550 approximately parallel to the piedmont relation but with an intercept of 48 2 0 8 mw m 2 these two relations may represent different heat flow provinces in the southeastern united states or the linear relations may simply be a consequence of differences in the thicknesses of the upper heat producing layer at least as far east as postmetamorphic granites sampled under the coastal plain

heat flow in the gulf coastal plain forty four new heat flow values from panhandle florida southwestern georgia southern alabama mississippi and louisiana range from 13 mw m sup 2 0 3 hfu 1 hfu 1 mu cal cm sup 2 s sup 1 41 8 mw m sup 2 in florida and alabama to 88 mw m sup 2 2 1 hfu in northern louisiana in general the values depict the eastern gulf coastal plain as an area with low to normal continental heat flow that increases from east to west anomalously low heat flow 13 33 mw m sup 2 0 3 0 8 hfu characterizes northcentral florida southwestern georgia and southern alabama while in northern louisiana 13 measurement sites yield values ranging from 50 mw m sup 2 1 2 hfu to 88 mw m sup 2 2 1 hfu distinctive disruptions in the temperature depth profiles attributed to convective effects within aquifers are limited in magnitude and extent and do not alter the geothermal gradient computations the region of relatively high heat flux in northern louisiana is coincident with an area underlain by cenozoic alkalic igneous rocks of the monroe uplift and an abnormal concentration of radiogenic heat sources within that body is postulated as the cause of the thermal anomaly

life on land in the proterozoic evidence from the torridonian rocks of northwest scotland the stoer group and diabaig formation of the torridonian succession in northwest scotland are late mesoproterozoic to early neoproterozoic ca 1200 1000 ma features preserved on the top surfaces of fine to medium grained sandstone beds in a number of stratigraphically and geographically separated localities are attributable to microbially induced sedimentary structures these include wrinkle structures remnants of apparent microbial crusts and indications of original cohesiveness and pliancy in sand sized sediment the surfaces on which the microbial structures formed were exposed subaerially abundant deep desiccation cracks and locally pedogenic structures in alluvial interfluve and lacustrine margin settings and many of the structures developed in areas well away from the perennially wetted regions adjacent to shorelines and fluvial channels thus these features indicate that earth9s biosphere had adapted to and colonized land surfaces many hundreds of millions of years before the dawn of the phanerozoic

proliferation of early triassic wrinkle structures implications for environmental stress following the end permian mass extinction lower triassic strata in the western united states and northern italy have yielded the first reported occurrence of microbially mediated wrinkle structures in shallow subtidal siliciclastic paleoenvironments since the cambrian we hypothesize that wrinkle structures formed under reduced infaunalization conditions during the aftermath of the end permian mass extinction these wrinkle structures occur in siliciclastic sedimentary rocks in association with hummocky cross stratification and in some cases trace fossils such as asteriacites rhizocorallium planolites and gyrochorte that indicate a subtidal paleoenvironment wrinkle structures were found in the campil member in the werfen formation of northern italy and in the virgin limestone member of the moenkopi formation and the upper member of the thaynes formation exposed in the western united states the presence of wrinkle structures is likely related to a unique taphonomic window that allowed for the preservation of these delicate features on storm dominated siliciclastic shelves wrinkle structures commonly formed in a variety of subtidal paleoenvironments during the proterozoic cambrian but thereafter became restricted to intertidal supratidal and deep sea environments this restriction has been attributed to the increase in infaunalization of metazoans following the cambrian radiation the proliferation of wrinkle structures in subtidal settings during the early triassic suggests that infaunal bioturbation was reduced after the end permian mass extinction and that this reduction lasted for millions of years

is there an archean crust beneath chesapeake bay a crescent shaped terrane of continental crust centered on chesapeake bay is interpreted as being a collisional relic of archean crust that was probably accreted during late hercynian times arcuate gravity and magnetic anomalies which represent the possible suture separate avalonian carolinian terranes from what seems to be african like rocks seen in boreholes in the east chesapeake bay region seismic and heat flow data also suggest an important lithological variation in basement type across the suture

microbial silicification in iodine pool waimangu geothermal area north island new zealand implications for recognition and identification of ancient silicified microbes silicified microbes provide evidence for some of the earliest life forms on earth they are extremely important to understanding the early development of life and the conditions that allowed its development such discussions commonly rely on comparisons with extant taxa and therefore depend upon the preservation style of the microbes and in particular the preservation of the taxonomically important features silicified microbes are deceptive they commonly appear to be well preserved even though their taxonomically critical features have been destroyed by silicification an understanding of the early taphonomic processes that influence microbial silicification can be obtained by studying extant microbes that are being silicified in modern hot spring pools iodine pool located in the waimangu geothermal area on the north island of new zealand is ideal for this purpose the spring water has a temperature of 69 100 c a ph of 8 3 9 0 and 440 457 ppm sio 2 glass slides left in the shallow marginal waters of this pool for 90 h became covered with thin layers of opaline silica discrete opal a spheres unsilicified microbes pseudofilaments partly silicified microbes and silicified microbes the rapidly silicified microbes appear well preserved with their general morphology diameter length and presence or absence of septa being readily apparent most of the silicified microbes however lack the key features that would allow accurate comparisons with extant taxa only two of the silicified microbes can be tentatively allied with the unsilicified forms despite being found side by side on the same glass slide these problems in identifying modern rapidly silicified microbes suggest that identifications of ancient silicified microbes can be problematic and must be treated with caution

chronological correlations between the pilbara and kaapvaal cratons abstract the early geological development of the pilbara and kaapvaal cratons has many features in common attempts have been made to correlate geologically similar features of the two cratons and it has been postulated that they originated as contiguous components of a single continent vaalbara during this time the early geological histories of the pilbara and kaapvaal cratons are here compared in detail and the evidence that they were initially contiguous is assessed these comparisons indicate significant differences in the chronologies of magmatic events within the granite greenstone crusts of the pilbara and kaapvaal cratons in addition igneous correlatives emplaced during ca 2985 and 2782 ma magmatic events on the kaapvaal craton have not been identified on the pilbara craton and a well defined 2760 ma magmatic event manifest as widespread emplacement of granitic rocks into the pilbara granite greenstone basement and eruption of flood basalts of the lower part of the fortescue group is absent from the kaapvaal craton furthermore similarities in first and second order transgression regression cycles within the sedimentary supracrustal sequences may be attributable to global sea level fluctuations and thus may be irrevelant to the question of former contiguity however similarities in some aspects of the geological development of the pilbara and kaapvaal cratons imply that there were periods extending for between 60 and 200 ma of the archaean era during which the style of crust formation intensity of volcanism and subaerial erosion and magnitude of sea level fluctuations may have varied on a global scale such similarities include the overall duration of formation of the granite greenstone crusts from ca 3650 to 3100 ma the onset of craton wide erosion in the interval ca 3125 to 3000 ma the major episodes of flood basaltic volcanism between 2760 and 2680 ma the predominance of chemical carbonate and banded iron formation sedimentation between ca 2630 and 2440 ma and the transition to widespread clastic sedimentation within the interval 2440 to 2200 ma

architectural elements from lower proterozoic braid delta and high energy tidal flat deposits in the magaliesberg formation transvaal supergroup south africa abstract three architectural elements are identified in the lower proterozoic magaliesberg formation pretoria group transvaal supergroup of the kaapvaal craton south africa 1 medium to coarse grained sandstone sheets 2 fine to medium grained sandstone sheets and 3 mudrock elements both sandstone sheet elements are characterised by horizontal lamination and planar cross bedding with lesser trough cross bedding channel fills and wave ripples as well as minor desiccated mudrock partings double crested and flat topped ripples due to the local unimodal palaeocurrent patterns in the medium to coarse grained sandstone sheets they are interpreted as ephemeral braid delta deposits which were subjected to minor marine reworking the predominantly bimodal to polymodal palaeocurrent trends in the fine to medium grained sandstone sheets are inferred to reflect high energy macrotidal processes and more complete reworking of braid delta sands the suspension deposits of mudrocks point to either braid delta channel abandonment or uppermost tidal flat sedimentation the depositional model comprises ephemeral braid delta systems which debouched into a high energy peritidal environment around the margins of a shallow epeiric sea on the kaapvaal craton braid delta and tidal channel dynamics are inferred to have been similar fine material in the magaliesberg formation peritidal complexes indicates that extensive aeolian removal of clay does not seem applicable to this example of the early proterozoic

kinematics of the secondary eyewall observed in hurricane rita 2005 abstractairborne doppler radar data collected from the concentric eyewalls of hurricane rita 2005 provide detailed three dimensional kinematic observations of the secondary eyewall feature the secondary eyewall radar echo shows a ring of heavy precipitation containing embedded convective cells which have no consistent orientation or radial location the axisymmetric mean structure has a tangential wind maximum within the reflectivity maximum at 2 km altitude and an elevated distribution of its strongest winds on the radially outer edge the corresponding vertical vorticity field contains a low level maximum on the inside edge which is part of a tube of increased vorticity that rises through the center of the reflectivity tower and into the midlevels the secondary circulation consists of boundary layer inflow that radially overshoots the secondary eyewall a portion of this inflowing air experiences convergence and supergradient forces that cause the air to rise and flow radially outward back into th

an overview of the geology of the transvaal sequence and bushveld complex south africa the late archaean early proterozoic transvaal sequence is preserved within the transvaal kanye and griqualand west basins with the 2050 ma bushveld complex intrusive into the upper portion of the succession within the transvaal basin both transvaal and bushveld rocks are extensively mineralized the former containing large deposits of iron manganese asbestos andalusite gold fluorine lead zinc and tin ores and the latter some of the world s major occurrences of pge chromium and vanadium ores transvaal sedimentation began with thin predominantly clastic sedimentary rocks black reef vryburg formations which grade up into a thick package of carbonate rocks and bif chuniespoort ghaap taupone groups these lithologies reflect a carbonate bif platform sequence which covered much of the kaapvaal craton in reaction to thermal subsidence above ventersdorp aged rift related fault systems an erosional hiatus was followed by deposition of the clastic sedimentary rocks and volcanics of the pretoria postmasburg segwagwa groups within the three basins under largely closed basin conditions an uppermost predominantly volcanic succession rooiberg group loskop formation is restricted to the transvaal basin a common continental rift setting is thought to have controlled pretoria group sedimentation rooiberg volcanism and the intrusion of the mafic rocks of the rustenburg layered suite of the bushveld complex the dipping sheets of the rustenburg magmas cut across the upper pretoria group stratigraphy and lifted up the rooiberg lithologies to form the roof to the complex subsequent granitic rocks of the lebowa and rashoop suites of the bushveld complex intruded both upper rustenburg rocks and the rooiberg felsites

paleontology of proterozoic shales and mudstones examples from the belt supergroup chuar group and pahrump group western usa abstract proterozoic black shales and other fine grained siliciclastic rocks contain a variety of fossils these fossils are illustrated by examples from three proterozoic units in the western usa the belt supergroup middle proterozoic the chuar group late proterozoic and the upper pahrump group late proterozoic a variety of microfossils and megafossils occur in the belt supergroup western montana acritarchs and filamentous microfossils are moderately well preserved in sub greenschist facies black shale in the little belt mountains the biologic affinity of the acritarchs is uncertain they could represent the vegetative or encystment stage of eukaryotic algae or the outer envelope of colonial coccoid cyanobacteria most of the filaments in these shales are of a size and morphology that is consistent with prokaryotic affinities however rare 20 and 40 m wide filaments could be either prokaryotic or eukaryotic higher grade sub greenschist facies dark gray to black mudstones in glacier national park contain poorly preserved acritarchs and spheroidal and filamentous pseudomicrofossils they are of interest because they provide information useful for distinguishing authentic microfossils from pseudofossils in mudstones and shales that have undergone substantial burial metamorphism several authentic megafossils occur in lower belt shales in the little belt and big belt mountains montana one of these is the carbonaceous compression beltina danai which could represent either fragments of a microbial mat or fragments of a sheet like megascopic alga the megafossil grypania spiralis has the form of a coiled ribbon and tentatively is interpreted as a megascopic alga green to purple mudstones and muddy sandstones in the lower belt supergroup in glacier national park contain markings that resemble a string of beads they are interpreted as megafossils that may have eukaryotic algal affinities black shales in the late proterozoic kwagunt formation chuar group grand canyon arizona contain a variety of acritarchs and filamentous microfossils of special interest is the vase shaped microfossil melanocyrillium which is interpreted as the sporangium of a eukaryotic and probably megascopic alga melanocyrillium also occurs in black mudstone in the late proterozoic pahrump group southeastern california

suspected microbial mat related crack like sedimentary structures in the palaeoproterozoic magaliesberg formation sandstones south africa abstract the occurrence of patchy preservation of ripples and of palimpsest ripples on many sandstone bed surfaces of the c 2 1 ga magaliesberg formation pretoria group transvaal supergroup south africa suggests that microbial mats grew within the upper parts of sandy deposits of the braid deltaic tidally controlled epeiric marine coastline palaeoenvironment inferred for this stratigraphic unit recently limited occurrences of petee ridges cracked sand layers elephant skin textures wrinkle structures and manchuriophycus like cracks have been found substantiating the envisaged role for microbial mats binding sandy sediment and providing cohesion during active sedimentation by fluvial and tidal processes ripple marks associated with most of these biogenic sedimentary structures enable evaluation of genetic processes for the ripples currents waves wind as well as estimation of wave heights and water depths cf tanner w f 1967 ripple mark indices and their uses sedimentology 9 89 104 tanner w f 1971 numerical estimates of ancient waves water depths and fetch sedimentology 16 71 88 the petee ridges are interpreted as reflecting spring low tidal desiccation which cracked the mats followed by disruption of sand beneath the mat due to loading from incoming tides or subsequent deposits slightly sinuous sand cracks on ripple crests are related to a similar combination of low tidal desiccation and formation of wind ripples with subsequent high tides providing the deeper water conditions and larger wave heights calculated from the tanner w f 1971 numerical estimates of ancient waves water depths and fetch sedimentology 16 71 88 formula synaeresis cracking is inferred for both elephant skin cracks and manchuriophycus structures these inferred suspect microbial sedimentary structures when studied in relation to data derived from associated ripples thus enable better estimation of depositional conditions within the magaliesberg palaeoenvironment and indicate minimum epeiric tidal ranges of c 112 cm in general these structures are found associated with evidence for shallower water and higher energy conditions which probably protected the microbial mats from degradation by heterotrophic bacteria through fast burial a possible association of such mat related structures with epeiric marine or passive margin coastline conditions may be pertinent for much of the precambrian clastic record

wrinkle marks an intertidal sedimentary structure due to aseismic soft sediment loading abstract the structures have a wide distribution intertidally in the macrotidal severn estuary and inner bristol channel occurring on the active sedimentary surface chiefly in association with a millimetre scale sand over mud doublet the sand surface is marked by depressions and ridges of a wide variety of shapes and orientations relative to slope that are matched antithetically by ridges and depressions on the mud sand interface below the sand layer consequently takes the form of a series of touching to detached pillow shaped masses whose centres lie 2 4 sand layer thicknesses apart wrinkle marks originate soon after the emergence of the sedimentary surface into the atmosphere and are considered to be soft sediment deformation structures on account of their 1 occasional displacement and or severance of early emergence surface organic trails and feeding marks 2 similarity in shape and internal structure to pseudo nodules recorded from the field and produced experimentally and 3 association with gravitationally unstable sediment layers in settings that favour the vigorous seepage of pore water through water logged sediments owing to their origin at the sedimentary surface wrinkle marks appear to be preserved mainly as surface markings on eroded and discordantly reburied mud sand interfaces

the effect of lateral deformation on heat flow heat production relations deformation that takes place following the fractionation of heat producing elements can cause the relationship of surface heat flow and heat production to deviate from a simple linear trend lateral transport by low angle normal faulting or thrust faulting may place the near surface rocks above rocks with different thermal properties even for a simple model the relationships can be complex but some constraints can be placed on the vertical distribution of heat generation and layer thickness measurements at many sites in a province could produce an overall linear trend of surface flux and heat production departures from linearity may be found at sites that were disturbed transported following the latest cycle of radioelement fractionation the amount of departure from linearity is a function of the thickness of the transported layer and the thermal properties of underlying layers

earth s earliest microbial mats in a siliciclastic marine environment 2 9 ga mozaan group south africa this study provides evidence for the existence of filamentous cyano bacteria forming sediment stabilizing mats in shelf environments at 2 9 ga the oldest known occurrence of microbial mats in siliciclastic rocks the mesoarchean mozaan group south africa features fine grained quartzites of an ancient shallow shelf environment these sandstones contain wrinkle structures which in thin section reveal filamentous textures forming carpet like microbial mat fabrics the textures resemble the trichomes of modern cyanobacteria chloroflexi or sulfur oxidizing proteobacteria mineralogical geochemical and isotopic analyses acre consistent with a biological origin of the filament like textures carbon filaments with biogenic isotopic signatures 1 3 c 24 2 0 5 are closely associated with hematite goethite and chert minerals which may derive from the former presence of oxygen within the microbial mats detrital quartz zircon and rutile in the mats could indicate baffling trapping and binding of the bacterial communities

microbial signatures in peritidal siliciclastic sediments a catalogue a catalogue of microbial structural signatures is presented based upon the coupling of fundamental biogeochemical microbial processes and local morphogenetic determinants it summarizes a collection of sedimentary structures obtained from two modern siliciclastic peritidal environments in different climatic zones temperate humid mellum island southern north sea subtropical arid coast of southern tunisia textural geometries reveal a high structural diversity but their determinants are primarily based upon six major parameters 1 intrinsic biofactors structural diversification of sedimentary microbial films and mats inherent in the organisms i e their construction morphology growth taxis and behaviour and local abundance of specific morphotypes most prominent are the ensheathed filamentous cyanobacteria microcoleus chthonoplastes and lyngbya aestuarii and the sheathless filamentous cyanobacterium oscillatoria limosa 2 biological response to physical disturbances sediment supply erosion and fracturing of surface layers resulting from desiccation cause growth responses of biofilms and microbial mats 3 trapping binding effects physicobiological processes give rise to grain orientations and wavy to lenticular lamina lamina specific grain arrangements and sucrose calcium carbonate accumulations 4 secondary physical deformation of biogenic build ups mechanical stresses acting upon sediments overgrown and biostabilized by biofilms and mats produce erosional and overthrust structures 5 post burial processes textural fabrics that evolve from mechanical effects of gas formation from decaying mats and features related to the formation of authigenic minerals calcium carbonates calcium sulphates pyrite 6 bioturbation and grazing post depositional structures such as skolithos type dwellings traces of burrowing insects gastropod grazing traces and faecal pellets in synopsis the catalogue firstly comprises a sound set of ubiquitous signatures this uniformity in architectural characteristics is attributed to the presence and local dominance of certain microbes throughout the different settings the catalogue secondly documents signatures that are extremely sensitive to tidal position hydrodynamic regime and overall climatic conditions these kinds of signature indicate narrow facies zones which often coincide with the activity or dominance zones of certain organisms an overview of structures of microbial origin from the fossil record underlines the potential of many of the signatures included in this catalogue to become fossilized and provide strong indicators of former siliciclastic tidal settings

sedimentation rates basin analysis and regional correlations of three neoarchaean and palaeoproterozoic sub basins of the kaapvaal craton as inferred from precise u pb zircon ages from volcaniclastic sediments abstract calculation of sedimentation rates of neoarchaean and palaeoproterozoic siliciclastic and chemical sediments covering the kaapvaal craton imply sedimentation rates comparable to their modern facies equivalents zircons from tuff beds in carbonate facies of the campbellrand subgroup in the ghaap plateau region of the griqualand west basin transvaal supergroup south africa were dated using the perth consortium sensitive high resolution ion microprobe ii shrimp ii dates of 2588 6 ma and 2549 7 ma for the middle and the upper part of the nauga formation indicate that the decompacted sedimentation rate for the peritidal flat to subtidal below wave base stratifera and clastic carbonate facies southwest of the ghaap plateau at prieska was of up to 10 m ma when not corrected for times of erosion and non deposition dates of 2516 4 ma for the upper gamohaan formation and 2555 19 for the upper monteville formation indicate that some 2000 m of carbonate and subordinate shale sedimentation occurred during 16 ma to 62 ma on the ghaap plateau for these predominantly peritidal stromatolitic carbonates decompacted sedimentation rates were of 40 m ma to over 150 m ma bubnoff units the mixed siliciclastic and carbonate shelf facies of the schmidtsdrif subgroup and monteville formation accumulated with decompacted sedimentation rates of around 20 b for the kuruman banded iron formation a decompacted sedimentation rate of up to 60 b can be calculated thus for the entire examined deep shelf to tidal facies range archaean and phanerozoic chemical and clastic sedimentation rates are comparable four major transgressive phases over the kaapvaal craton followed by shallowing upward sedimentation can be recognized in the prieska and ghaap plateau sub basins in griqualand west and partly also in the transvaal basin and are attributed to second order cycles of crustal evolution first order cycles of duration longer than 50 ma can also be identified the calculated sedimentation rates reflect the rate of subsidence of a rift related basin and can be ascribed to tectonic and thermal subsidence comparison of the calculated sedimentation rates to published data from other archaean and proterozoic basins allows discussion of general precambrian basin development siliciclastic and carbonate sedimentation rates of archaean and palaeoproterozoic basins equivalent to those of younger systems suggest that similar mechanical chemical and biological processes were active in the precambrian as found for the phanerozoic particularly for stromatolitic carbonates matching modern and neoarchaean sedimentation rates are interpreted as a strong hint of a similar evolutionary stage of stromatolite building microbiota the new data also allow for improved regional correlations across the griqualand west basin and with the malmani subgroup carbonates in the transvaal basin the nauga formation carbonates in the southwest of the griqualand west basin are significantly older than the gamohaan formation in the ghaap plateau region of this basin but are in part correlatives of the oaktree formation in the transvaal and of parts of the monteville formation on the ghaap plateau

the sequence stratigraphic concept and the precambrian rock record an example from the 2 7 2 1 ga transvaal supergroup kaapvaal craton abstract sequence stratigraphic concepts are applied to the 2 7 2 1 ga transvaal supergroup which constitutes the sedimentary floor to the bushveld igneous complex the transvaal succeeds the foreland fill of the witwatersrand supergroup and in its lowermost portions is partly synchronous with the ventersdorp supergroup lavas that followed upon the witwatersrand sediments in the stratigraphic record the unconformable contacts at the base and the top of the transvaal supergroup mark significant changes in the overall tectonic setting which qualifies them as first order sequence boundaries sedimentation within the transvaal basin was controlled by cycles of extensional and or thermal subsidence resulting in the accumulation of sequences separated by stages of base level fall generating sequence bounding subaerial unconformities the transvaal first order sequence is subdivided into five second order unconformity bounded depositional sequences i e the protobasinal black reef chuniespoort rooihoogte timeball hill and boshoek houtenbek sequences the span of time of these second order sequences averaging 130 ma is much longer than the duration of the second order phanerozoic cycles which is explained by the less evolved and thus slower plate tectonic processes that dominated the late archaean early proterozoic times each transvaal second order sequence preserves a variable number of systems tracts mainly as a function of the strength of the erosional processes associated with the ravinement surfaces and subaerial unconformities in a complete succession a sequence would include a basal lowstand systems tract lst followed by transgressive tst highstand hst and falling stage fsst systems tracts the protobasinal and rooihoogte timeball hill sequences only preserve the lst and tst due to the strong post depositional erosion the black reef sequence is composed of lst tst and hst the chuniespoort sequence preserves the tst hst and fsst but lacks the lst due to ravinement erosion the boshoek houtenbek sequence develops the complete succession of systems tracts common features may be emphasized between the same types of systems tract developed within different sequences the lst includes high energy coarse prograding facies such as alluvial fans and fan delta systems that fill the irregularities of the pre existing topography and lead to the peneplanation of the depositional areas the tst may be recognized from retrogradational stacking patterns that are associated with the transgression of marine environments the hst marks the normal regression of the previous marine environments which assumes coeval aggradation of fluvial and basinal facies within an overall progradational framework the fsst would normally include only basinal facies age equivalent with the correlative subaerial unconformities in our case study the stages of base level fall resulted invariably in fully nonmarine environments within the transvaal area generally leading to the development of subaerial unconformities in particular circumstances the nonmarine environment may still accumulate and preserve fluvial alluvial fan or lacustrine sediments as a function of the relative position between topography and local equilibrium profiles and base levels the interpretations of base level rise and fall resulting from the application of the sequence stratigraphic concept to the 2 7 2 1 ga transvaal supergroup enable estimation of sea level changes during deposition of this succession with an understanding of local tectonic conditions on the kaapvaal craton and of coeval sedimentation elsewhere in africa eustatic and relative sea level changes and variation in continental freeboard conditions may be suggested for the transvaal supergroup

mat related sedimentary structures in neoproterozoic peritidal passive margin deposits of the west african craton anti atlas morocco abstract proterozoic inliers in the central anti atlas mountains expose predominantly siliciclastic sedimentary successions deposited in peritidal zones along the neoproterozoic continental margin of the west african craton wac the low grade metamorphic and modestly deformed sediments contain a wealth of sedimentary structures related to the former presence and activities of microbial mats and respective physicobiological processes the well preserved structures include wrinkle structures erosion marks microbial sand chips spindle shaped and subcircular microbial shrinkage cracks and possibly gas domes and cabbage head structures thin sections exhibit mat fragments and dispersed grains of hematite limonite after pyrite in fine grained quartzitic storm deposits post storm layers frequently consist of matrix supported sand sized to silt sized grains and are overlain by argillaceous veneers including isolated silt sized grains and black carbonaceous laminae the muddy veneers are considered to represent compacted stacks of microbial mats biolaminites which colonized and biostabilized storm and post storm layers and thus prevented them from eroding in the absence of grazing and burrowing organisms and at suitable depositional and hydrodynamic conditions it may be expected that proterozoic microbial mats extensively grew from the supratidal to the intertidal zones and occasionally e g behind protective barriers in the subtidal zone and beyond mat related structures however need specific conditions for their formation and preservation wrinkle structures erosion marks and microbial sand chips require tractional currents and soon deposition and burial respectively such conditions are preferably met in intertidal and supratidal zones spindle shaped and subcircular cracks require mat shrinkage due to either desiccation or syneresis crack propagation implies progressive shrinkage while superposition of crack generations indicates repeated alternation between mat exposure and flooding respective conditions prevail in the upper intertidal zone gas domes and cabbage head structures are related to the production of gas from decaying organic material beneath a sealing cohesive mat they may be some of the first features formed during genesis of petee structures in the intertidal to lower supratidal zones mat related structures may serve as sensitive facies indicators once their modes of formation are revealed

a sulfur isotope study of pyrite genesis the mid proterozoic newland formation belt supergroup montana abstract different generations of sedimentary pyrite from the mid proterozoic newland formation usa have been analysed for their sulfur isotopic compositions the results indicate bacterial sulfate reduction as the pyrite forming process the 34 s values for early diagenetic pyrite around 14 are in contrast to dominantly more positive values for many other middle proterozoic units a progressive reduction of sulfate availability during diagenesis can be recognized by an increase in 34 s content rayleigh distillation as well as through detailed petrographic observations contemporaneous seawater had a sulfur isotopic ratio between 14 and 18 as measured from sedimentary barite within the unit