Microbial ooids and cortoids from the Lower Triassic (Spathian) Virgin Limestone

Microbial ooids and cortoids from the Lower Triassic (Spathian)Virgin Limestone,Nevada,USA:Evidence for an Early Triassic microbial bloom in shallow depositional environments

Adam D.Woods

Department of Geological Sciences,California State University,Fullerton,800N.State College Blvd.,Fullerton,CA 92834-6850,USA

a b s t r a c t

a r t i c l e i n f o Article history:

Received 20June 2011

Received in revised form 12June 2012Accepted 23July 2012

Available online 21August 2012Keywords:Early Triassic ooids cortoids microbialite biotic recovery

Lower Triassic sedimentary rocks contain a variety of unusual facies and fabrics,with microbialites being a distinctive component of many carbonates deposited following the Permian –Triassic mass extinction.Coated grains are common in shallow water facies from the upper Lower Triassic (Spathian)Virgin Limestone (Moenkopi Formation)in southern Nevada,and were investigated in order to determine their origin.Petro-graphic analysis reveals that the majority of the coated grains found within the Virgin Limestone are micritic ooids with a concentric fabric,or with a homogenous fabric composed of dense,often cloudy micrite.In ad-dition,asymmetric ooids,aggregate grains,and distorted ooids are also locally common in some oolitic units;low-Mg calcite ooids and bimineralic ooids composed of low-Mg calcite and dense,cloudy micrite are less commonly found,but are also documented from the Virgin Limestone.

Cortoids (i.e.,grains that are coated with constructive micrite envelopes)are a minor component of oolitic grainstones and packstones (typically 10–15%of the grains),although they may also comprise entire beds.The cortoids are coated with micrite sim-ilar to that which comprises the ooid cortices,and may be ?nely laminated or dense and cloudy in nature.The micrite ooids and constructive micrite envelopes are interpreted as microbial in origin based on the ?nely laminated or cloudy,dense nature of the micrite,as well as coatings that are uneven,or often of greater thick-ness on one side of elongate nuclei,such as bivalve shells or phylliod algae blades.The origin of the low-Mg calcite ooids and layers is less certain,but may also be microbial.The results of this study suggest that a mi-crobial bloom occurred in shallow water environments,which was the result of 3factors:(1)the unusual chemistry of Early Triassic oceans;(2)runoff of nutrient-rich waters,which enhanced microbialite growth;and,(3)wave agitation and warm waters that led to CO 2degassing and further supersaturation of shallow waters with respect to calcium carbonate.

1.Introduction

The aftermath of the Permian –Triassic mass extinction was one of the most unusual periods in Earth history,with an array of sedimentologic,paleontologic,and geochemical indicators signifying an unsettled,com-plex world following the biotic crisis that lasted until the beginning of the Middle Triassic.Evidence of anomalous oceanic conditions during the Early Triassic include indicators of widespread anoxia in the global oceans (e.g.,Hallam,1991;Wignall and Hallam,1993;Isozaki,1997;Wignall and Twitchett,2002a;Grice et al.,2005;Takahashi,2009),elevat-ed levels of CO 2in the surface ocean (Woods et al.,1999,2007;Pruss et al.,2005;Knoll et al.,2007),and evidence of sustained high primary produc-tivity (Algeo and Twitchett,2010;Meyer et al.,2011).Multiple occur-rences of sea ?oor precipitates (e.g.,Baud et al.,1997;Woods et al.,1999,2007;Heydari et al.,2003;Kershaw et al.,2011),microbialites (e.g.,Baud et al.,1997,2005,2007;Sano and Nakashima,1997;

Kershaw et al.,1999,2011;Lehrmann,1999;Ezaki et al.,2003,2008;Pruss and Bottjer,2004b;Pruss et al.,2004,2005;Mary and Woods,2008;Pruss and Payne,2009;e.g.,Schubert and Bottjer,1992;Wignall and Twitchett,2002b;Woods and Baud,2008;Shen et al.,2010)and ?at pebble conglomerates (Wignall and Twitchett,2002b;Pruss et al.,2005)further imply that Early Triassic seawater chemistry was favorable to the abiotic and microbial precipitation of calcium carbonate.Finally,rapid,substantial swings in carbon isotopes across the Early Triassic peri-od corroborate the unsettled nature of the period (Payne et al.,2004;Corsetti et al.,2005).The result of anomalous oceanic conditions was to delay or dampen recovery in those regions where unfavorable environ-mental conditions were present (Hallam,1991;Schubert and Bottjer,1995;Payne et al.,2004;Pruss and Bottjer,2004a;Bottjer et al.,2008);however,life recovered quickly when severe environmental conditions ameliorated (Krystyn et al.,2003;Twitchett et al.,2004;Chen et al.,2007;Beatty et al.,2008;Zonneveld et al.,2010).

Ooid-rich grainstones have been documented extensively near the Permian –Triassic boundary,where they are often associated with microbialites or synsedimentary sea ?oor cements (e.g.,Heydari et al.,

Global and Planetary Change 105(2013)91–101

E-mail address:awoods@https://www.360docs.net/doc/c614000232.html, .

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2003;Baud et al.,2005;Algeo et al.,2007;Groves et al.,2007;Haas et al., 2007;Lehrmann et al.,2007;Kershaw et al.,2011).In some cases,low-ermost Lower Triassic ooids are anomalously large(e.g.,Payne et al., 2006;Lehrmann et al.,2007;Weidlich,2007),and are thought to pro-vide further evidence of the unusual carbonate chemistry of the oceans following the Permian–Triassic mass extinction(Groves et al.,2003; Groves and Calner,2004).Lower Triassic ooids have not been extensive-ly documented or described above the boundary interval,which is a potentially signi?cant gap in our understanding of Lower Triassic car-bonate systems and the role abiotic and microbial processes played in an unusual post-extinction carbonate system.

Ooids and coated grains are a common component of shallow subtidal and intertidal carbonates of the Lower Triassic(Spathian)Virgin Lime-stone of southern Nevada and southwestern Utah,and are especially abundant east of Las Vegas(Skyllingstad,1977),where they may com-prise up to50%of the rock(Larson,1966).These grains have been docu-mented in MS and PhD theses(Larson,1966;Skyllingstad,1977;Shorb, 1983),but have not been examined in any great detail.Ooids and coated grains from the Virgin Limestone are of interest given their abundance as well as previous hypotheses that many of the coated grains are microbial in origin(Skyllingstad,1977;Shorb,1983).Furthermore,the occurrence of microbialites(Schubert and Bottjer,1992;Pruss and Bottjer,2004b; Pruss et al.,2005;McCoy and Woods,2009;Pruss and Payne,2009; Mata and Bottjer,2011)and sea?oor precipitates(Woods et al.,1999, 2007)in laterally equivalent facies suggests that the ooids and coated grains may also be related to the anomalous nature of Early Triassic oceans,and warrants further,detailed examination.Therefore,the aims of this study are to:(1)document and describe ooids from the Virgin Limestone;and,(2)determine the origin of the ooids and establish their relationship,if any,to the unusual chemistry of Early Triassic oceans.

2.Geologic background and study localities

The Virgin Limestone is a member of the Moenkopi Formation, which is composed of a series of inter?ngering terrestrial red bed,evap-orite,and marine carbonate units.The Virgin Limestone is Spathian in age(Fig.1A)based on ammonoid biostratigraphy(Poborski,1954) and strontium isotopes(Marenco et al.,2008).The Virgin Limestone was part of a larger,distally steepened carbonate ramp system(Stone et al.,1991),referred to as the Moenkopi Platform by Woods(2009),de-posited along the western margin of Pangaea(Fig.1B).The Moenkopi

Platform transitions from peritidal and shallow subtidal facies of the Virgin Limestone in eastern Nevada and Utah to middle and inner car-bonate ramp environments of the Virgin Limestone in western Nevada (Larson,1966;Reif and Slatt,1979;Woods,2009)to outermost carbon-ate ramp and basinal environments of the Union Wash Formation in east-central California(Stone et al.,1991).To the east,in Arizona,the Moenkopi Platform passes into extensive sabkha facies and?uvial red bed facies(Reif and Slatt,1979).

Ooids and coated grains from shallow subtidal facies of the Virgin Limestone were examined from2primary localities in southern Nevada (Fig.1B):(1)Horse Spring Valley(Figs.1B and2)and(2)Ute(Figs.1B and3).In total,69thin sections and hand samples from the Ute locality and39thin sections and hand samples from the Horse Spring locality were examined.Additional thin sections and hand samples were exam-ined from shoaling facies near the upper contact of the Virgin Limestone with the Schnabkaib Member of the Moenkopi Formation at Lost Cabin Spring,NV in order to provide additional petrographic examples where needed,or to corroborate interpretations from the primary study localities.

3.Microbialites from peritidal and shallow subtidal facies of the Virgin Limestone

Analysis of the hand samples and thin sections from peritidal and shallow subtidal facies of the Virgin Limestone reveals a variety of microbially coated grains,including micrite ooids and cortoids (allochems with constructive micrite coatings),as well as agglutinated microbialites composed of coated grains and peloids.In addition,aggre-gate grains,asymmetric ooids and distorted ooids(i.e.,ooids deformed by compaction)were also documented.

3.1.Micrite ooids

Ooids are a common component of shallow water facies of the Virgin Limestone and are readily observed in outcrop(Fig.4A).The ooids range from spherical to ellipsoidal in shape,and are typically0.25–0.75mm in diameter,although ooids as large as2mm in diameter have been noted (Skyllingstad,1977).Ooids are typically found in packstones and grainstones;however,micritic limestone intraclasts will occasionally con-tain ooids?oating in matrix.Skeletal grains with constructive micritic en-velopes(cortoids)are often found with the ooids(comprising10–15%of the allochems),but they will be considered separately.

Thin section analysis reveals that the cortices of the ooids are com-posed of micrite with an internal fabric that ranges from distinctly concentrically laminated to homogenous and dense(Fig.4B).Differ-ences in internal fabric are likely the result of where the plane of the thin section cut through the ooid,with more oblique cuts resulting in less well-de?ned laminae.Individual lamina within concentrically-laminated ooids ranges from10to25μm thick (Fig.4C).The laminae are composed of alternating layers of equal or

Interbedded

Limestone and Shale

Fig.1.A)Lower Triassic stratigraphy for the Moenkopi Formation(Larson,1966).Ammo-noid biozones(Tozer,1994)and conodont biozones(Orchard and Tozer,1997)from west-ern Canada.P,Proptychites;V,Vavilovites;Anawasatch,Anawasatchites;K,Keyserlingites; S,Silberlingites;N,Neospathodus;Ic,Icriospathodus;Neogond,Neogondolella.B)Early Triassic paleogeography and distribution of Lower Triassic outcrops in southern Nevada.LCS=Lost Cabin Spring,Nevada;U=Ute,Nevada;HSV=Horse Spring Valley,Nevada;NV=Nevada; AZ=Arizona;UT=Utah;CA=California;OR=Oregon;ID=Idaho.

Panel B is modi?ed from Marzolf,1993.

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near equal thickness of dark micrite and light micrite that includes clear microspar crystals(Fig.4C).The laminae may be distinct and continuous;however,the laminae often become less evident or even disappear laterally(Fig.4D).Concentric micrite laminae are typically parallel to each other;however,asymmetric ooids(i.e.,cortex is thicker on one side)frequently occur(Fig.4E).Ooids are often cemented together to form aggregate grains;the cements binding the grains are made up of laminated micrite or homogenous micrite similar to that coating the ooids(Fig.4F).

Ooid nuclei fall into2main groups.The majority of the ooid nuclei consist of aggregates of anhedral to subhedral sparry calcite crystals that together comprise50–75%of the volume of the ooid(Fig.4C). The crystals found in the nuclei vary in size from10to250μm across and are composed of clear calcite with occasional pyrite coatings along crystal boundaries.These nuclei are almost always round in thin section,but oblong nuclei of this type occasionally occur (Fig.5A).Ooid nuclei less commonly consist of intraclasts that are composed of cloudy micrite(Fig.5B)or an irregular mixture of micrite and microspar;these nuclei may also be round in thin section, but are more commonly oblong and result in ellipsoidal ooids. Skeletal grains rarely serve as ooid nuclei,but include ostracodes, foraminifera,and mollusk shell fragments.

Several oolitic limestones from both the Horse Spring Valley locality and the Ute locality contain low-Mg calcite ooids or ooids with rims or internal layers composed of low-Mg calcite(Fig.6A,B).Low-Mg calcite ooids typically comprise b10%of the limestones at both localities,but make up~50%of the ooids in a thin limestone bed found90m above the base of the section at the Horse Spring Valley locality.The low-Mg calcite is honey-colored in thin section and is composed of acicular crystals oriented in a radial fabric.The low-Mg calcite layers often have a patchy appearance due to the inclusion of micrite,and patchy zones frequently alternate with clear zones(Fig.6A).In some cases, very thin(5–10μm)internal laminae of dense micrite occur that impart a concentric fabric(Fig.6C).The micrite found in bimineralic grains is typically homogenous,cloudy,and dense(Fig.6B).

Distorted ooids are found in2units at the Ute locality(Fig.3).The distorted ooids occur in layers1–3mm thick and along the periphery of mudstone intraclasts(Fig.7A).The ooids are typically sheared into sigmoid-shaped bodies(Fig.7B),and may be stretched to the point where they form continuous layers of zigzag shaped grains.The ooids frequently appear to have undergone brittle deformation along their outer edges that ruptured their cortices(Fig.7C).

3.2.Constructive micrite envelopes

Skeletal grains coated with layers of micrite(cortoids)that range in thickness from0.01to0.25mm,are commonly found within the Virgin Limestone at both study localities(Fig.8A).Cortoids are often found within oolitic packstones and grainstones,but they comprise a minor fraction of the allochems,typically around10–15%,although pockets or beds in which cortoids comprise a majority of the allochems are not uncommon.Evidence for a constructive origin for the micrite enve-lopes,as opposed to a destructive origin in which endolithic boring de-stroys the internal structure of a grain and replaces it with featureless micrite(Bathurst,1964,1966),is as follows:(1)the micrite envelopes appear to be building out from the grain surface(Fig.8B);(2)the contact between the micrite envelope and the allochem is smooth,as opposed to the irregular contacts and microborings associated with

Fig.2.Stratigraphic section of the Virgin Limestone at the Horse Spring Valley,NV locality.93

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destructive micritization (Fig.8C);(3)the micrite envelopes are typi-cally of varying thickness,and often are thicker on one side of the coated allochem than the other (Fig.8A);and,(4)the micrite envelopes do not appear to be substrate-dependent:a variety of types of grains exhibit constructive micrite envelopes,including intraclasts (Fig.8D).The micrite that comprises the envelopes is similar to the micrite that comprises ooid cortices,and may be ?nely laminated,with laminae,which are composed of layers of light and dark micrites that range from 10to 25μm thick (Fig.8C)or the micrite envelopes may be dense and cloudy (Fig.8A,D).3.3.Agglutinated microbialites

The Horse Spring Valley locality contains a number of beds that are made up of an amalgamation of allochems,including peloids,oncoids,ooids,intraclasts,and rare skeletal grains,which are interpreted to be coarse-grained agglutinated microbialites.These units are often laminat-ed at the 2–3mm-scale,and have an open,fenestral fabric (Fig.9A).Peloids (100–120μm in diameter)are a very common component of these beds,comprising 50%or more of the allochems.Oncoids are the second most common grain,comprising ~40%of the grains,with ooids,intraclasts and skeletal grains making up the remaining ~10%of the allochems (Fig.9B).The oncoids are coated with alternating light-and dark-colored laminae that are similar in appearance to the cortices of the concentric ooids discussed above,and are of approximately the same thickness (10–25μm thick)(Fig.9B).Oncoids and ooids can be dif-?cult to distinguish from each other since both grains are similar inter-nally,and asymmetric ooids are prevalent in the microbialites (Fig.9B).Many of the allochems are distorted,leading to frequent concavo-convex contacts between the grains (Fig.9C).

4.Discussion

Micrite ooids can form as the result of complete destructive micritization of ooids,growth of micrite crystals in environments with low sedimentation rates (e.g.,outer ramp environments),or due to the precipitation of micrite within bio ?lms (Flügel,2010).Formation of the ooids found in the Virgin Limestone through

destructive micritization is unlikely based on the excellent preservation of the con-centric fabric;the internal fabric of the grains is typically destroyed if the ooids have been micritized (senso Bathurst,1964,1966).Growth of the micrite ooids as the result of reduced sedimentation rates is also improbable based on the shallow subtidal to intertidal depositional environment,the input of terrigenous clays,silts,and ?ne sands via ?u-vial input from the east (Reif and Slatt,1979),and the large volume of skeletal remains and lime mud found throughout the Virgin Limestone (Larson,1966).The micrite ooids,therefore,are interpreted to have formed from the microbially-mediated precipitation of micrite,which is supported by laminae that are often more distinct or less distinct lat-erally in the concentric micrite ooids as well as the cloudy,dense micrite that comprises the cortices of the homogenous ooids (Flügel,2010).Other factors indicative of a microbial origin for the ooids are the forma-tion of asymmetric ooids in low energy environments as well as within agglutinated microbialites (see below)and the nature of the micrite coatings around cortoids (discussed below).Concentric ooids and ho-mogenous ooids may have formed under differing energy regimes resulting in contrasting cortex structures;alternatively,the difference may be related to differences in the makeup of microbial communities that caused micrite precipitation.

The ooids formed under high energy conditions on beaches and off-shore bars (Reif and Slatt,1979)as well as in tidal deltas (Skyllingstad,

MS WS PS GS

Shale

S l t s t V F S s F S s M S s

C S s V C S s

Fig.3.Stratigraphic section of the Virgin Limestone at the Ute,NV locality.

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1977),where the ooids were frequently moved in order to develop their spherical or rounded shape and resulted in the deposition of oolitic packstones and grainstones.Ooids are also found in offshore tempestites (at the Ute locality)and in washover fans (i.e.,thin oolitic grainstones surrounded by shales or mudstones),suggesting that the grains were highly mobile.If the grains were swept into low energy environments,continued microbially mediated micrite precipitation on static grains led to the formation of asymmetric ooids (Freeman,1962;Gasiewicz,1984)as well as the formation of aggregate grains (Tucker and Wright,1990).

The large,sparry calcite nuclei of many of the ooids are composed of spar crystals that vary greatly in size (10–250μm),and are randomly arranged,which argues against an aragonite precursor,in which the spar crystals are frequently arranged in a tangential brickwork pattern and are of similar sizes (Assereto and Folk,1976;Wilkinson et al.,1984,1985;Singh,1987).In addition,bivalve,gastropod,and phylloid algae blades found in the same thin sections as the low-Mg calcite ooids and coatings have been replaced by equant spar (refer to Fig.8A,C);these grains should retain their original microfabric if low-Mg calcite were replacing aragonite or high-Mg calcite at a very detailed level (e.g.,Wilkinson et al.,1984).Large,hollow nuclei have been docu-mented from ooids and coated grains forming around gas bubbles (e.g.,Schreiber et al.,1981;Gerdes et al.,1994).Such a scenario for the large nuclei for many of the ooids from the Virgin Limestone

A D

E F

Fig.4.Micrite ooids from the Virgin Limestone.A)Outcrop photograph of an oolitic grainstone.The ooids are an abundant allochem within the Virgin Limestone east of Las Vegas and may comprise up to 50%of the rock (Larson,1966).Scale bar=10mm.B)Micrite ooids in thin section.The ooids are made up of micrite with an internal fabric that ranges from concentrically laminated to homogenous and cloudy or https://www.360docs.net/doc/c614000232.html,minated and homogenous ooids often occur together,as in this photo,and likely represent differences in where the plane of the thin section cut across the ooid.The ooids typically have large nuclei that may comprise 50–75%of the volume of the ooid.Scale bar=500μm.C)Ooid laminae are made up of alternating dark micrite and clear microspar laminae that are equal or nearly equal in thickness.Scale bar=100μm.D)Ooid laminae may be distinct and continuous across individual grains;however,the laminae often become less distinct and may disappear altogether (arrows).Scale bar=250μm.E)Asymmetric ooids.Note that the additional calcite that was added to the cortex is homogenous and cloudy.Scale bar=250μm.F)Aggregate grain.The ooids and other grains are frequently cemented together to form aggregate grains that may be bound together by laminated micrite (as in this example)or homogenous dense to cloudy micrite.Scale bar=250μm.

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seems improbable;however,since these allochems are typically found in grainstones or packstones that are frequently cross-bedded or hum-mocky cross-strati ?ed and were therefore deposited in depositional settings that would have been subjected to continuous or episodic bursts of high energy.Such conditions would be expected to break grains with hollow centers and lead to a greater percentage of broken ooids.A more likely scenario is that the ooid nuclei were replaced by sparry calcite cement during diagenesis.One possible material that may have served as nuclei is ripped-up pieces of microbial mats,which may act as ooid nuclei in modern environments (e.g.,Gerdes et al.,1994).Because the mats were composed primarily of organic material,they would have decayed away during early diagenesis,leading to the formation of a void that was later replaced by spar,or a micritic nucleus if the mats contained a large percentage of lime mud.Further evidence for the ooids having a soft,organic cen-ter is the deformed ooids found at the Ute locality,in which the unlithi ?ed or stiff micritic nucleus underwent plastic deformation during shallow burial that stretched the ooids and resulted in brittle breakage of the outer cortex (Fig.7B,C).Kershaw et al.(2011)docu-mented ooids with nuclei composed of single crystals of calcite from low-ermost Triassic carbonates from the ?ürük Dag section in southern Turkey,and suggested that crystals originated within synsedimentary sea ?oor cements.Similar cements are common in the deeper water equivalent of the Virgin Limestone,the Union Wash Formation of east-central California (Woods et al.,1999,2007;Pruss et al.,2005;

Woods,2009);however,it seems unlikely that broken crystals from sea-?oor cements served as ooid nuclei given that similar sea ?oor cements have not been documented from the Virgin Limestone,which was depos-ited in shallower depositional

environments.

Fig.5.Ooid nuclei.A)The nuclei of the majority of ooids from the Virgin Limestone are composed of sparry calcite crystals that range in size from 10to 250μm across.Sparry calcite crystals in the nuclei of ooids often have pyrite coatings along the crystal boundaries.Thin section from the Horse Spring Valley,NV locality.Scale bar=500μm.B)The ooid nuclei are less commonly made of intraclasts of cloudy micritic limestone.Thin section from the Horse Spring Valley,NV locality.Scale bar=500μm.

Fig.6.The low-Mg calcite ooids and layers.A)A low-Mg calcite ooid from the Horse Spring Valley,NV locality composed of honey-colored,low-Mg calcite crystals in a radi-al fabric.The patchiness of the interior layer is probably due to the inclusion of micrite during growth.Scale bar=500μm.B)Bimineralic ooids with low-Mg calcite rims or internal layers from the Horse Spring Valley,NV locality.Scale bar=250μm.C)Low-mg calcite ooid from the Ute,NV locality with very thin (5–10μm)internal laminae of dense micrite.Scale bar=250μm.

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Low-Mg calcite ooids are rare today,even in terrestrial environments, but have been documented from shallow,moderate energy zones along the edges of freshwater lakes(e.g.,Wilkinson et al.,1980;Davaud and Girardclos,2001).Like today,the Early Triassic was a period of aragonite seas(Sandberg,1983),when high Mg/Ca favored the precipitation of aragonite and high-Mg calcite.The Virgin Limestone was deposited under arid conditions(Reif and Slatt,1979),which would have driven Mg/Ca ratios higher locally via gypsum precipitation in sabkhas and sali-nas,and made low-Mg calcite precipitation even less likely.The low-Mg calcite ooids and layers could be the result of diagenetic replacement of aragonite or high-Mg calcite by low-Mg calcite,but this seems unlikely given the highly-detailed level of preservation that would be required, and a lack of a similar degree of preservation for bivalves,gastropods, and phylloid algae blades found in the same thin sections(Wilkinson et al.,1984).Another possibility is that the low-Mg calcite ooids and coatings formed in?uviotidal channels where freshwaters led to the growth of low-Mg calcite and the intermittent input of marine waters via tides led to the precipitation of the microbial layers in the bimineralic grains;however,this hypothesis also seems unlikely given that there is no evidence of this type of system having existed in the region;?uvial ooids require the input of Ca-enriched water,typically from springs (e.g.,Geno and Chafetz,1982).Given the microbial origin of the micrite ooids,the most likely source of the low-Mg calcite ooids and coatings is also through microbial activity,possibly due to differences in microbial communities or processes;however,this hypothesis will require further testing through future work.

The micrite coatings on the cortoids are constructive in origin,in which micrite was precipitated onto the surface of the grain,possibly through the precipitation of micrite within bio?lms(Perry,1999),or the calci?cation of cyanobacterial?laments on grain surfaces(Kobluk and Risk,1977).Further evidence for a microbial origin for the constructive micrite envelopes is the often-unequal thicknesses of the envelopes on opposite sides of oblong grains,suggesting that the envelopes grew faster on one side than another.The similar appearance,internal structure,and thicknesses of micrite envelopes and ooid cortices suggest that the micrite ooids and constructive micrite envelopes had the same origin.Small per-centages of cortoids in oolitic grainstones and packstones are re?ective of a smaller volume of skeletal grains or intraclasts present in ooid shoals, beaches,or tidal deltas that could be coated with constructive micrite envelopes.

The agglutinated microbialites likely formed in areas that were typi-cally quiet based on the asymmetric and irregular shape of many of the oncoids and ooids found there,along with the presence of aggregate grains.Some grains may have precipitated directly within microbial mats,similar to what occurs in some environments today(Gerdes et al., 1994);however,it appears that many of the grains were trapped by a sticky mat,and then continued to grow,resulting in irregularly-shaped ooids and oncoids.Some of the grains were poorly lithi?ed,leading to the deformation of less coherent grains during burial.The grains that comprise the oncoids and ooids in the agglutinated microbialites have a similar internal structure to the ooid cortices and constructive micrite en-velopes coating the cortoids,providing further evidence of an analogous, microbial origin for each.

5.An Early Triassic microbial bloom

Mata and Bottjer(2011)note that microbialites within subtidal fa-cies of the Virgin Limestone are associated with maximum?ooding sur-faces and are rare elsewhere.The results of this study demonstrate that microbially-mediated precipitation of calcite was very common in shal-low subtidal and intertidal environments regardless of shifts in sea level.The great volume of ooids and common cortoids in grainstone and packstone units is indicative of a microbial bloom in shallow waters that can be attributed to3factors:(1)the unusual seawater chemistry of the Early Triassic;(2)the in?ux of large volumes of nutrients via terres-trial runoff and possibly upwelling;and,(3)the uniformitarian processes that lead to the precipitation of carbonate in shallow,tropical waters today.

Widespread anoxia in Early Triassic oceans has long been proposed for Early Triassic oceans,and is supported by multiple lines of evidence (e.g.,Hallam,1991;Wignall and Hallam,1993;Isozaki,1997;Wignall and Twitchett,2002a;Grice et al.,2005;Wignall et al.,2005,2010; Takahashi,2009),although recent studies suggest that anoxia may have been limited to the oxygen minimum zone(Algeo et al.,2010). Nonetheless,widespread anoxia led to the development of alkaline deep waters over time via sulfate reduction of organic matter that drifted down from surface waters(Kempe,1990;Grotzinger and Knoll,1995; Woods et al.,1999).Upwelling of deeper,anoxic and alkaline waters led to CO2-degassing and supersaturation of the remaining waters with respect to calcium carbonate,and the growth of sea?oor precipitates and microbial carbonate in deeper waters(Kempe,1990;Beukes and Klein,1993;Grotzinger and Knoll,1995;Woods et al.,1999,2007; Pruss et al.,2005).Environmental stresses would be limited to regions near the chemocline;however,the carbonate chemistry of waters across the shelf would continue to have a greater degree of supersaturation with respect to calcium carbonate(Grotzinger and Knoll,1995).Super-saturation of Early Triassic waters may have been further augmented by a decline in the uptake of carbonate by metazoans following the Permian–Triassic mass extinction(Groves et al.,2003;Groves and Calner,2004).

Enhanced continental weathering during the Early Triassic likely in-creased the amount of terrestrial-derived nutrients to the continental

Fig.7.Distorted ooids from the Ute,NV locality.A)The distorted ooids occur in layers 1–3mm thick and along the periphery of mudstone intraclasts.Scale bar=2mm.

B)The distorted ooids are frequently squeezed into sigmoid-shaped bodies.Scale bar=250μm.C)The ooids frequently show evidence of brittle deformation along their edges as suggested by nuclei and inner cortices that are deformed and outer cortices that are broken.Scale bar=250μm.97

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shelves(Algeo and Twitchett,2010)and resulted in high rates of prima-ry productivity(Suzuki et al.,1998;Takahashi et al.,2009;Algeo and Twitchett,2010;Algeo et al.,2010;Meyer et al.,2011).An increase in nutrients via?uvial input and possibly upwelling would also lead to an increase in the production of microbial carbonate in nearshore envi-ronments,which would be further intensi?ed by the removal of CO2 during photosynthesis.High primary productivity driven by terrestrial nutrient runoff is likely for the Virgin Limestone given that the unit intertongues with?uvial red beds(Reif and Slatt,1979),while en-hanced weathering of the continents led to an increase in the delivery of nutrients to the oceans(Algeo and Twitchett,2010).

Calcium carbonate is strongly supersaturated in modern tropical shallow-water environments where warm waters and wave agitation lead to enhanced supersaturation of calcium carbonate due to the loss of CO2(Tucker and Wright,1990).The microbial ooids were generated in high-energy shoals,beaches,and tidal deltas(Skyllingstad,1977;Reif and Slatt,1979)in a tropical setting,possibly during a period of elevated global temperatures(Retallack,1999),which would also increase the degree of supersaturation of calcium carbonate,and further enhance the production of microbial carbonate.

Why then,are microbialites not common across the entire ramp? Microbialites are rare in the middle and outer parts of the ramp,and are typically only associated with maximum?ooding surfaces,when the importation of deep,alkaline and anoxic waters led to the enhanced growth of microbial and abiotic carbonate and dampened grazing pres-sures via environmental stress(Mata and Bottjer,2011).A lack of micro-bial carbonate on much of the ramp suggests that grazing pressures were strong enough to keep microbial growth in check.Beatty et al.(2008) proposed the idea of a shallow marine“habitable zone”,where harsh en-vironmental conditions that were present in deeper waters were amelio-rated as atmospheric oxygen was added to shallow waters via stirring by waves.Under improved conditions biotic recovery was able to take place (e.g.,Twitchett et al.,2004),and grazing pressures quickly returned.An-other possibility is that nearshore environments were stressful,and this allowed microbialite growth by dampening grazing pressures.Local eutrophication due to nutrient-driven productivity may have discour-aged grazers,especially in lagoons,while high rates of evaporation in an arid environment may have altered salinities close to shore. Kershaw et al.(2012)noted the importance of local conditions in the growth of microbial carbonate in sedimentary rocks deposited immedi-ately following the Permian–Triassic mass extinction,and the results of this study suggest that local conditions continued to play an important role in microbialite growth.

6.Conclusions and broader implications

Shallow subtidal and intertidal facies of the Virgin Limestone contain abundant ooids and coated grains that are interpreted to be microbial in origin.The ooids formed on beaches,offshore bars and tidal deltas; asymmetric ooids and aggregate grains formed when the grains were swept into quiet environments,and were sometimes incorporated into agglutinated microbialites.The results of this study are indicative of a mi-crobial bloom close to shore that was the result of several factors,includ-ing the unusual chemistry of Early Triassic oceans,terrestrial runoff of nutrient-rich waters that enhanced primary productivity,and the warm,agitated nature of the shallow waters.These ooids were deposited during the Spathian,and as such,represent one of the last occurrences of unusual sedimentary features following the Permian–Triassic mass ex-tinction.Future work will determine the relative importance of each fac-tor in the growth of the micrite ooids and cortoids;however,local conditions may have been the most important aspect given that ooids have not been extensively documented above the lowermost Lower Tri-assic,directly above the Permian–Triassic boundary.Ooids have been noted from the Dienerian–Smithian(?)Gastropod Oolite Member of the Werfen Formation of northern Italy(Broglio Loriga et al.,1990; Nützel and Schulbert,2005)and from the Upper Olenekian–Lower Anisian Mongila Formation of the western Balkanides in Bulgaria (Chatalov,2005a,b).The ooids from the Gastropod Oolite Member of the Werfen Formation are similar in appearance to those from the Virgin Limestone,and may have a microbial origin(Nützel and Schulbert,

A B

Fig.8.Cortoids.A)Skeletal grains,like the bivalve in the center of the photo,are coated with layers of micrite that range in thickness from0.01to0.25mm.Coatings may be even around the edge of the grain,or uneven,as in this photo,which is indicative of constructive micrite growth on the grain as opposed to destructive micritization.Ute,NV locality. Scale bar=3mm.Further evidence for a constructive origin of the micrite envelopes includes:B)The micrite envelopes build out from the grain surface,given a more rounded shape. Ute,NV locality.Scale bar=1mm.C)The contact between the micrite envelope and the allochem is smooth;irregular contacts and microborings during destructive micritization lead to irregular boundaries.Ute,NV locality.Scale bar=250μm.D)The micrite envelopes are not dependent on the substrate:a variety of types of grains exhibit micrite envelopes,including intraclasts.Horse Spring Valley,NV locality.Scale bar=250μm.Note that the micrite that makes up the envelopes is similar to the micrite that comprises ooid cortices in Fig.4.Construc-tive micrite envelopes can be?nely laminated,with laminae composed of layers of dark micrite and clear microspar that range from10to25μm thick or dense or cloudy.

98 A.D.Woods/Global and Planetary Change105(2013)91–101

2005),while the ooids from the Mongila Formation are interpreted to be bimineralic aragonite-high-Mg calcite ooids.More work is necessary,therefore,to establish the distribution and origin of Lower Triassic ooids deposited above the Permian –Triassic boundary,and to determine their relationship to both global and local phenomena at work during this most unusual period in Earth history.

Acknowledgments

The author wishes to thank Paul Alms,Scott Mata,Jen McCoy,and Kathleen Ritterbrush for ?eld support and useful discussions in the ?eld.Scott Mata provided the measured stratigraphic column used in the ?eld and in this paper for the Ute locality.Isabel Monta?ez provided help with thin sections as well as useful advice on the low-Mg calcite ooids and coatings.David Bottjer,Steve Kershaw,and Yongbiao Wang provided helpful comments that improved the manuscript.

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