AUTHORS

Daniel J. Lehrmann � Department ofGeoscience, Trinity University, San Antonio, Texas;dlehrmann@trinity.edu

Daniel Lehrmann received his bachelor’s degreefrom the University of Wisconsin Oshkosh, his M.S.degree from the University of Wisconsin Madison,and his Ph.D. from the University of Kansas. Heworked as a research geologist for Exxon Produc-tion Research from 1993 to 1996. From 1996 to2010, he was a professor at the University of Wis-consin Oshkosh. He is currently the Pyron Pro-fessor of Geoscience at Trinity University in SanAntonio.

Marcello Minzoni � Shell International Ex-ploration and Production Company, Houston,Texas; M.Minzoni@shell.com

Marcello Minzoni received his bachelor’s andM.S. degrees from the University of Ferrara in

GEOLOGIC NOTE

Lower Triassic oolites ofthe Nanpanjiang Basin,south China: Faciesarchitecture, giant ooids,and diagenesis—Implicationsfor hydrocarbon reservoirsDaniel J. Lehrmann, Marcello Minzoni, Xiaowei Li,Meiyi Yu, Jonathan L. Payne, Brian M. Kelley,Ellen K. Schaal, and Paul Enos

Italy. He received his Ph.D. from the Universityof Kansas. He is currently a senior research andexploration geologist at Shell International Ex-ploration and Production in Houston.

Xiaowei Li � Department of Resources andEnvironmental Engineering, Guizhou University,Caijiaguan, Guizhou Province, People’s Republicof China

Li Xiaowei received his bachelor’s degree with amajor in environmental geology from GuizhouUniversity in China. He recently completed his M.S.degree (2011) in geology and geochemistry fromGuizhou University. He is planning to continuegraduate work in geosciences in the United States.

Meiyi Yu � Department of Resources and En-vironmental Engineering, Guizhou University,Caijiaguan, Guizhou Province, People’s Republicof China; yuyouyi@163.com

Yu Meiyi received his bachelor’s degree in sciencefrom the Wuhan College of Geology. He workedas a geologist for the Regional Mapping Team ofthe Guizhou Bureau of Geology and Mineralresources from 1985 to 2000. He is currentlyassistant professor at Guizhou University inGuiyang, Guizhou Province, China.

Jonathan L. Payne � Department of Geologi-

ABSTRACT

Lower Triassic platforms in the Nanpanjiang Basin contain ex-tensive oolites. Interior oolites are stacked in meter-scale cy-cles arranged into larger coarsening-upward sequences.Oolitesthicken toward margins to include grainstones up to 50 m(164 ft) thick and contain giant ooids (up to 1 cm [0.4 in.]) andcomposite coated grains. Cross-bedding, ripples, and abradedooids indicate deposition in high-energy shoals. Apparentlayer-cake correlation across interiors indicates amalgamationof shoals. Thinner interior lenses represent spillover lobes.

Ooids are interpreted to have originally been bimineralicwith cortices of radial or micritic fabrics (high-magnesium cal-cite), alternating with coarse pseudospar or brickwork (origi-nally aragonite). Distorted ooids formed by brittle compactionof micritic cortices around voids are interpreted to have beendissolved aragonite. Abundant potential nuclei indicate thatlimited supply was not a factor contributing to the large ooidsize. High-energy and abnormally high–seawater CaCO3 sat-uration are interpreted to be causes of the giant ooids. Mostprevious reports of giant ooids come from the Neoprotero-zoic, a period of increasing surface-water oxygenation and

cal and Environmental Sciences, Stanford Univer-sity, Stanford, California; jlpayne@stanford.edu

Jonathan Payne received his bachelor’s degreefrom Williams College and his Ph.D. from HarvardUniversity. He is currently an assistant professor

Copyright ©2012. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received September 30, 2011; provisional acceptance December 7, 2011; revised manuscriptreceived January 5, 2012; final acceptance January 23, 2012.DOI:10.1306/01231211148

AAPG Bulletin, v. 96, no. 8 (August 2012), pp. 1389– 1414 1389

of geological and environmental sciences atStanford University.

Brian M. Kelley � Department of Geologicaland Environmental Sciences, Stanford University,Stanford, California; bmkelley@stanford.edu

Brian Kelley received his bachelor’s degree fromKent State University. He is currently a Ph.D.candidate at Stanford University.

Ellen K. Schaal � Department of Geologicaland Environmental Sciences, Stanford University,Stanford, California; eschaal@stanford.edu

Ellen Schaal received her B.A. degree in geologyfrom Carleton College and is currently a Ph.D.candidate at Stanford University.

Paul Enos � Department of Geology, Univer-sity of Kansas, Lawrence, Kansas; enos@ku.edu

Paul Enos is an emeritus distinguished professorat the University of Kansas, where he taught from1982 to 2003 and also received a B.S. degreein geology in 1956. He earned an M.S. degreefrom Stanford University and a Ph.D. from YaleUniversity. From 1964 to 1970, he was a researchgeologist at Shell Development Company andwas a faculty at State University of New YorkBinghamton from 1970 to 1982.

ACKNOWLEDGEMENTS

This research is based on work supported by theNational Science Foundation (EAR-9804835 to D. J.Lehrmann and EAR-9805731 to P. Enos), the Pe-troleum Research Fund of the American ChemicalSociety (40948-B2 and 33122-B8 to D. J. Lehrmann,34810-AC8 and 37193-AC8 to P. Enos, and 45329-G8 to J. L. Payne), the National Geographic So-ciety (8102-06 to J. L. Payne), and the Shell Inter-national Exploration and Production (46000572to D. J. Lehrmann). The authors acknowledgethe logistical and geologic support of the GuizhouBureau of Geology, Guizhou University, and theGeological Survey of Guangxi. Qilong FuSee, ShaoQ. Sun, and Stephen N. Ehrenberg are acknowl-edged for reviews that substantially improvedthe manuscript.The AAPG Editor thanks the following reviewers fortheir work on this paper: Stephen N. Ehrenberg,Qilong Fu, and Shao Q. Sun.

1390 Geologic Note

highCaCO3 saturation caused by a minimal skeletal carbonateprecipitation. We interpret similar seawater chemistry in theaftermath of the end-Permian extinction to explain the genesisof the giant ooids in the Early Triassic. The genesis of bimi-neralic ooids during an Early Triassic period of rapidly in-creasing pCO2 and low SO2�

4 indicates that an increasing Ca/Mg ratio was the primary mechanism driving the change fromaragonite to calcite seas.

The architecture, textures, and diagenesis of the LowerTriassic oolites of the Nanpanjiang Basin provide useful analogsfor coeval reservoirs in Sichuan and the Middle East.

INTRODUCTION

Oolite is a carbonate sediment or sedimentary facies composedof concentrically coated calcium carbonate grains arbitrarilydefined as being less than 2 mm in diameter and formed pri-marily in shallow-marine environments (Tucker and Wright,1990).Oolites have received a great deal of attentionwith studiesinto the petrography and environmental controls on genesisgoing back to Sorby (1879) and include studies of modern andancient oolites. Petrographic studies have focused on the size,cortical fabrics, mineralogy, function of microbes, and modes ofdiagenetic alteration (cf. Sandberg, 1975; Wilkinson andLanding, 1978; Tucker, 1984; Green et al., 1988, among manyothers).

Numerous studies have focused on determining the origi-nal mineralogy of marine ooids, with the goal of constrainingthe composition of ancient seawater (Sandberg, 1975, 1983,1985; Wilkinson and Landing, 1978; Wilkinson and Given,1986; Wilkinson et al., 1985). Ooids may form with originalcortices composed of low magnesium (Mg) calcite, high Mgcalcite, or aragonite. Although highMg calcite and aragonite arereplaced by low Mg calcite in most ancient rocks, remnant orig-inal fabrics, destructive replacive fabrics, mineral inclusions, andgeochemical analysis can be used with a fairly high level ofconfidence in identifying original mineralogies (Sandberg,1985). Oolites with original aragonite cortical composition canbe recognized by dissoluton and compaction of ooids duringearly diagenesis and burial. Ooids with nuclei or cortical lam-inae composed of aragonite are preferentially removed by dis-solution, resulting in early compaction and collapse (distortedooids) (Wilkinson and Landing, 1978; Wilkinson et al., 1983).In bimineralic ooids, the most commonly observed fabricconsists of an inner radial high–Mg calcite cortical layer fol-lowed by micritic or tangential aragonite cortical laminae or

alternating tangential or micritic and radial cor-tical layers (Wilkinson et al., 1983; Tucker, 1984;Chow and James, 1987; Major et al., 1988). Severalstudies have verified bimineralic ooids geochemi-cally using high levels of strontium (Sr) as a proxyfor aragonite and Mg for high Mg calcite. Marineooid cortical structures with well-preserved radialormicritic fabricswere originally composed of highMg calcite, whereas cortical layers were composedof coarse pseudospar, and those with dissolutionand compacted fabrics or brickwork spar textureswere originally composed of aragonite (Wilkinsonet al., 1983; Tucker, 1984; Chow and James, 1987;Major et al., 1988; Swett andKnoll, 1989;Chatalov,2005, among others).

Oolites have been reported from marine car-bonate strata ranging across the Phanerozoic to theProterozoic and Archean (Sandberg, 1983; Tucker,1985; Wilkinson et al., 1985; Swett and Knoll,1989; Wright and Altermann, 2000). The originalmineralogy of ooids andmarine cements have variedsystematically through geologic history, indicatingfirst-order secular changes in seawater chemistrybetween aragonite and calcite seas (Sandberg, 1983;Wilkinson et al., 1985; Hardie, 1996). These changesreflect alternations between major icehouse periodsof Earth history characterized by a cool global cli-mate, extensive ice caps, and lowglobal sea level andgreenhouse periods characterized by a warm globalclimate, reduced ice caps, and continents floodedby epicontinental seas (Read, 1998).

Oolites form some of the best carbonate hy-drocarbon reservoirs. Examples include the Juras-sic Smackover oolites of the U.S. Gulf Coast andthe Jurassic Arab D reservoirs of the Middle East,which host some of the world’s largest hydro-carbon accumulations (cf. Lindsay et al., 2006).Permian–Triassic oolite facies also form significanthydrocarbon reservoirs of the Khuff Formation inthe Middle East and the Feixianguan Formation ofthe Sichuan Basin (Alsharhan, 2006; Ehrenberget al., 2007; Ma et al., 2007; Peng et al., 2010).

A grain-size boundary of 2 mm was arbitrarilydefined to separate ooids from larger coated grainstermed “pisoids” (cf. Peryt, 1985). Although thisclassification is useful operationally, many oolitesinterpreted to have formed in shallow-marine shoals

in carbonate platforms slightly exceed the 2-mmcutoff, and pisoids (pisolites) are typically associ-ated with hypersaline or vadose settings (Tuckerand Wright, 1990). Thus, several authors workingwith shallow-marine oolites have referred to thecoated grains exceeding 2 mm as “oversized” or“giant ooids” to distinguish their interpreted modeof genesis from vadose pisoids (Swett and Knoll,1989; Sumner and Grotzinger, 1993; Husinec andRead, 2006). Giant ooids with diameters rangingup to 1.5 cm (0.6 in.) are most commonly reportedfrom the Precambrian. A high carbonate satura-tion state of the oceans (resulting from lack ofskeletal calcium carbonate precipitation), high en-ergy (resulting from increased tidal forces and prev-alence of ramps), and low supply of nuclei forooid growth (lack of peloids or skeletal debris) havebeen suggested as possible mechanisms underlyingthe prominence of giant ooids in the Precambrian(Sumner and Grotzinger, 1993). Oversized ooidshave also been reported from the Lower Triassicstrata of south China (Payne et al., 2006a; Li et al.,in press). Perhaps this is not surprising because theLower Triassic strata contain few skeletal grainsand a variety of anachronistic facies, such as mi-crobialites, which have been interpreted to reflectreduced biodiversity or anomalous ocean chemis-try in the aftermath of the end-Permian mass ex-tinction (cf. Lehrmann et al., 2003).

The purpose of this article is to document thefacies architecture, sedimentary features, and pe-trography of Lower Triassic oolite deposits in sev-eral carbonate platforms from the NanpanjiangBasin and to interpret their depositional environmentsand originalmineralogy. This study provides usefulconstraints on the environmental factors leadingto the widespread deposition of oversized or giantooids, and it provides outcrop analogs useful forcomparison with oolite hydrocarbon reservoirs inage-equivalent strata.

GEOLOGIC SETTING

During the Permian and the Triassic, the Nanpan-jiang Basin formed an embayment in the southernmargin of the Yangtze cratonic block (Figure 1).

Lehrmann et al. 1391

The Yangtze block separated from Gondwanalandduring the Paleozoic, moved northward across thePaleotethys and accreted to the North China blockduring theLateTriassic Indosinian orogeny (Figure 1,inset) (Enos, 1995; Meng and Zhang, 1999). Bor-dering the southernmargin of the Yangtze block arethe Ailaoshan and Songma suture zones (Figure 1).Although the timing is controversial, the Ailaoshanand Songma are interpreted to be zones of collisionwith the Simao (Burma) and Indochina blocks, re-spectively (Metcalfe, 1996; Lepvrier et al., 1997).

During the Early Triassic through the MiddleTriassic, the basin subsided rapidly as several car-bonate platforms developed within the basin, and

1392 Geologic Note

siliciclastic turbidites filled the basin from the lateEarly Triassic to the early Late Triassic. Many sci-entists interpret this to be a foreland phase of basindevelopment connected with convergence alongthe Songma suture zone to the south and/or alongthe Ailaoshan zone to the west during the Indo-sinian orogeny (Enos, 1995; Carter et al., 2001;Enos et al., 2006; Lehrmann et al., 2007). Con-vergence along the Songma is supported by pat-terns of greater and earlier subsidence, carbonateplatform stepback and drowning in the southernpart of the basin, and progressive thickening ofacidic volcanic ash horizons to the south (Newkirket al., 2002; Lehrmann et al., 2007).

Figure 1. Tectonic map illustrating the cratonic blocks (plates) of south China, the interpreted suture zones, and the extent of theNanpanjiang Basin and Yangtze platform. The South China block includes the Yangtze craton and the south China fold belt. Inset, upperright illustrates global plate reconstruction and the position of South China block (SC), the North China block (NC), and the Indochinablock (I) in the Late Permian. Modified from Lehrmann et al. (2007).

From the late Paleozoic through the MiddleTriassic, the Nanpanjiang Basin was surrounded onthree sides by a vast marine carbonate and silici-clastic shelf, the Yangtze platform. Isolated plat-forms developed within the basin, including theGreat Bank of Guizhou (GBG), Chongzuo-Pingguoplatform (CPP), and the Debao and Heshan plat-forms (Figure 2) (Lehrmann et al., 2007). Theoverall sequence stratigraphy, chronostratigraphy,and tectonic influences on platform evolution andarchitecture have been interpreted for several areasof the Yangtze platform,GBG, andCPP (Enos et al.,2006; Lehrmann et al., 2007; Minzoni, 2007).

The Yangtze platform and the isolated plat-forms evolved low-angle ramps or platform archi-tectures with marginal oolite shoals and peritidalinterior facies in the Early Triassic (Lehrmann et al.,2007). The southerly platforms drowned and wereburied with siliciclastic turbidites in the beginningof the Middle Triassic (early Anisian), with the ex-ception of small pinnacle platforms that developedin the northernmost part of the CPP. The Yangtzeplatform, GBG, and pinnacle platforms of theCPP evolved progressively steepening Tubiphytesboundstone reef margins and laterally variable ar-chitecture interpreted to have resulted primarily

Figure 2. Lower Triassic (Scythian) lithofacies and interpreted paleogeography of the Nanpanjiang Basin and Yangtze platform in partsof Guizhou, Guangxi, and Yunnan. Compiled from regional geologic maps of the Yunnan Bureau of Geology and Mineral Resources(1984), Guangxi Bureau of Geology and Mineral Resources (1985), and Guizhou Bureau of Geology and Mineral Resources (1987). Mapshave been modified with results from our mapping. Localities discussed in the text include the Bangeng (Bg), Bunong (Bn), Chongzuo-Pingguo platform (CPP), Dajing-Xiaojing (Dj), Debao platform (DB), Dawen (Dw), Great Bank of Guizhou (GBG), Guandao (Gd), Guohua(Gh), Guiyang (Gy), Hanlong (Hl), Heshan platform (HS), Hongyan (Hy), Longbang (Lb), Louhua (Lh), Liujiao (Lj), Longya (Ly), Taipingsection (Tp), Xiliang (Xl), Yongningzhen (Yn), and Zhenfeng (Zf).

Lehrmann et al. 1393

from differences in tectonic subsidence and acti-vation of syndepositional faults in the Middle Tri-assic (Enos et al., 2006; Lehrmann et al., 2007;Minzoni, 2007).

This article focuses on the oolitic limestonesand dolostones within the Lower Triassic ramp-platform margins and interiors of the isolatedplatforms and the Yangtze platform. Data con-trol comes from geologic maps and stratigraphicsections described by the Guizhou and Guangxigeologic surveys (Guangxi Bureau of Geologyand Mineral Resources, 1985; Guizhou Bureau ofGeology and Mineral Resources, 1987), field map-ping, reconnaissance observations, and stratigraphicsections. Among the isolated platforms, the greatestdata control has been developed for the GBG andCPP. Petrographic details come from the descrip-tion of polished slabs and5×7.6–cm (2×3–in.) thinsections prepared for oolite units from each of theplatforms.

FACIES ARCHITECTURE ANDDEPOSITIONAL ENVIRONMENTS

Oolites occur in the Majiaoling and Beisi forma-tions, of Induan and Olenekian age, respectively,in the isolated platforms of Guangxi (Figure 2).Mapping and stratigraphic sections of the CPPdemonstrate that oolites occur near the platformmargin and in depositional sequences that corre-

1394 Geologic Note

late across the platform interior (Figures 3, 4).Oolites of the Beisi Formation form distinctive,thick, cliff-forming units that dominate the mod-ern landscape in the isolated platforms of Guangxi(Figure 5A). The Majiaoling Formation is com-posed of thin-bedded recessive lime mudstone,with siliciclastic mudstone partings up to 3 cm(1.2 in.) thick, punctuated by a few decimeter- tometer-thick, massive, resistant oolite beds. The limemudstone is light gray, homogeneous, locally withbedding-plane burrows, bivalves, gastropods, andpeloids.Oolite units aremore abundant toward theplatform margin, whereas mudstone partings aremore abundant in the interior, reflecting a highercurrent energy at the margin than in the interior.Interior oolite beds commonly have a scoured baseand fine upward, suggesting that they were depos-ited as spillover lobes shed from the margin. In theCPP, the Majaoliing Formation ranges from 110 to140 m (361–495 ft) in thickness and contains lessthan 1% oolite in the interior and up to 11% oolitenear the margin (e.g., Ly section) (Figure 4).

In the CPP, the Beisi Formation is from 615 to775 m (2017–2542 ft) thick and contains from 20to 35% oolite by thickness. The oolite is packagedinto three or four major shallowing-upward de-positional sequences 100 to 250 m (328–820 ft)thick that correlate over 100 km (62mi) across theinterior of the CPP (Figure 4). The lower transgres-sive part of depositional sequences is dominatedby thin, platy-bedded, bioturbated, lime mudstone

Figure 3. Restored cross sections of the Chongzuo-Pingguo platform, illustrating the Late Permian through the Middle Triassic evolutionand sequence stratigraphy. Sections Liujiao (Lj), Longya (Ly), and Taiping (Tp) are presented in Figure 4; locations are shown in Figure 2.

with shale partings identical in character to theunderlying Majiaoling Formation (Figure 5B). Oo-lite beds punctuate the succession and progres-sively increase in number and thickness upward inthe depositional sequences. The oolites are grain-stone with planar lamination, cross-bedding, andherringbone cross-bedding, indicating depositionin high-energy shoals influenced by tidal currents(Figure 6A, B). In the middle and upper parts ofthe depositional sequences, the oolites occur in

smaller meter-scale depositional cycles that changeupward from thin-bedded lime mudstone to oolitegrainstone capped with flaser-bedded ribbon rock(Figure 4). The ribbon rock contains scours andasymmetrical ripples with reversing current di-rections alternating with lime mudstone drapesinterpreted to represent peritidal deposition (cf.Demicco andHardie, 1994; Lehrmann et al., 2001).Oolites in the upper part of the cycles are alsocommonly bioturbated packstone, indicating shoal

Figure 4. Stratigraphic cross section of the Chongzuo-Pingguo platform. Stratigraphic sections Liujiao (Lj), Longya (Ly), and Taiping(Tp) occur within the platform; Bunong (Bn) at the basin margin (locations shown in Figures 2, 4). The Tp section occurs nearly 100 km(62 mi) north of the Ly section. The SB-1 through SB-3 are sequence boundaries. Locations are shown in Figures 2, 3.

Lehrmann et al. 1395

stabilization. The uppermost parts of the deposi-tional sequences contain massive amalgamatedoolite units ranging from 25 to 50 m (82–164 ft)thick (Figures 4, 5C). The tops of some of themassive amalgamated oolite beds contain fenestral(or keystone) pores and meniscus cement, indicat-ing supratidal exposure.

Oolites in the interior of the CPP are domi-nantly lime grainstone, although they are dolomi-tized in the upper part of the Beisi Formation.Ooids in the interior sequences typically range from0.5 to 1 mm (Figure 6C). Rarely, interior oolitescontain ooids greater than 2 mm and include giantooids up to 5 mm in diameter and large intraclastsand composite coated grains up to 1.5 cm (0.6 in.).

Toward the platform margin, giant ooids (typ-ically 3–7 mm but ranging up to 1 cm [0.4 in.] indiameter) and composite coated grains up to 4 cm

1396 Geologic Note

(1.6 in.) dominate the succession (Figure 6D).Oolitic grainstones thicken toward the southernand northern margins of the CPP, where they dom-inate the succession (e.g., Guohe and Longbangareas on the north margin and Longya section onthe south margin) (Figures 2, 4). Interior and mar-gin oolites also contain interbeds of mollusk andpackstone, as well as peloidal-foraminiferal limemudstones.

The dominance of thick amalgamated oolitegrainstones and giant ooids at platform marginsdemonstrates that the oolites formed as high-energyshoals. The correlation of oolite units in large-scaledepositional sequences across vast areas of the in-terior of the CPP and the tabular resistant cliffsobserved on outcrops gives the appearance thatthe units correlate in layer-cake fashion across theplatform interior. However, it is unlikely that oolite

Figure 5. Outcrop photographs of shallow-marine carbonate facies of the Majiaoling and Beisi formations in the Chongzou-Pingguoplatform. (A) Panoramic view of oolites of the Beisi Formation in the Chongzuo area. Massive cliffs that dominate the landscape areamalgamated oolite grainstone beds up to 50 m (164 ft) thick. (B) Majiaoling Formation, thin-bedded lime mudstone (lower left) andBeisi Formation, with cliff-forming oolite units (upper right), lower part of the Longya (Ly) section. Cliff-forming units total approximately45 m (∼148 ft) thick. (C) Massive amalgamated ooid-grainstone units in the Beisi Formation. The Ly section, 237 m (777 ft). People forscale (circle, lower left).

shoals could exist synchronously across such a vastarea. Subtle differences in correlation of interioroolites and thickening toward the margin (Figure 4)indicate that they formed as the result of progra-dation and amalgamation of a complex of shoalsacross the interior. Reconnaissance observations in-dicate that similar oolite sequences occur through-out the interiors of the Debao and Heshan plat-forms in Guangxi (Figure 2). If this interpretationis correct and oolites occur across the interiors of allof the isolated platforms in the basin, they cover avast area—approximately 10,000 km2 (∼3844mi2)(exclusive of the Yangtze platform) (Figure 2).

InGuizhou Province, oolites occur in the LowerTriassic formations in the Great Bank of Guizhou,the northernmost isolated platform in the basin,and in the Yangtze platform that borders the ba-sin (Figure 2). In the Great Bank of Guizhou,oolites occur in the Lower Triassic platform in-terior and bank margin facies mapped as the Dayeand Anshun formations by the Guizhou Bureauof Geology (Figures 7–9). In the Yangtze plat-form, oolites occur in the Induan Daye and Yelang

formations (representing middle and inner ramp)and the Olenekian Anshun and Yongningzhen for-mations (representing platform margin and inte-rior facies) (Figures 10–12).

In the Great Bank of Guizhou, a dolomitizedoolitic unit 90 m (295 ft) thick correlates across thebanktop and is constrained as Induan–Dienerian inage on the basis of carbon isotope stratigraphycalibrated with biostratigraphy at the basin margin(Figure 7) (Payne et al., 2004; Kelley et al., 2011;Meyer et al., 2011). The lower 50 m (164 ft) is acoarsely crystalline ooid dolograinstone, commonlywith fabric destructive dolomitization, althoughlocal areas have microcrystalline dolomite and non-dolomitized and preserve fabrics with fine petro-graphic detail. Locally, cross-bedding and herring-bone cross-bedding are observed on weatheredsurfaces (Figure 8A). Although the unit is domi-nantly composed of ooids less than 0.7 mm indiameter, it also contains giant ooids up to 4 mmand composite ooids and rounded intraclasts upto 1.7 cm (0.7 in.) (Figure 8B). Cross-bedding inthe lower part of the Dienerian unit indicates

Figure 6. Macroscopic char-acteristics of oolites of theChongzuo-Pingguo platform(CPP). (A) Trough cross-beddingin oolite grainstone of the BeisiFormation. (B) Herringbonecross-bedding (reversing currentdirections) in oolite grainstone ofthe Beisi Formation. (C) Polishedslab of typical platform-interioroolite grainstone facies of theBeisi Formation. Grain size isabout 0.3 mm. From 299 m(981 ft) at the Liujiao (Lj) sec-tion. (D) Outcrop photograph ofgiant ooids typically 0.7 mm indiameter but ranging up to 1 cm(0.4 in.) and large coated intra-clasts ranging up to 2 cm (0.8 in.)across from the northern mar-gin of the CPP in the Pingguoarea.

Lehrmann et al. 1397

high-energy shoal environments. The upper 40 m(131 ft) of the Dienerian unit is an oolitic, crypt-algal, fenestral laminate. Ooids and large, rounded,tabular, ooid grainstone intraclasts occur trappedbetween micritic cryptalgal laminae. Fenestrae, dis-solved ooids, and meniscus cements indicate sub-aerial exposure on a supratidal flat (Figure 8C).The sediment was probably dolomitized in thesupratidal environment, as evidenced by micro-crystalline dolomite with exquisite fabric preser-vation. The superposition of the cryptalgal-oolitefacies over the cross-bedded oolite grainstone in-dicates progradation of tidal-flat facies over thetop of oolite shoals across the platform interior

1398 Geologic Note

(Figure 7). Akin to the CPP, the oolite most likelydid not develop synchronously across the vast areaof the bank top but instead developed by lateralprogradation and amalgamation of shoal and tidal-flat complexes.

Areas of the Lower Triassic margins of theGreat Bank of Guizhou are pervasively dolomi-tized, making it difficult to decipher the characterof the margins (Kelley et al., 2011). However, pres-ervation of oolite fabric on weathered surfaces ofdolostones (Figure 8D), margin areas preserved aslimestone (Figure 9), and oolite turbidites in ad-jacent basin margin facies demonstrate extensiveshoal development at the platformmargin. Platform

Figure 7. (A) Restored cross sections of the Early Triassic of the Great Bank of Guizhou (Figure 2 for location). For stratigraphic data,see Lehrmann et al. (1998). Sb = sequence boundary; Dw = Dawen; Dj = Dajiang. (B) Stratigraphic correlation of dolo-oolite across theplatform interior between the Dw and the Dj sections. The lower part is coarsely dolomitized cross-bedded grainstone interpreted torepresent amalgamated shoal deposits. The upper part is ooid bearing fenestral-cryptalgal laminites interpreted to represent adjacenttidal-flat deposits that prograded over the shoals. M = mudstone; W = wackestone; P = packstone; G = grainstone; B = boundstone.

margin ooid grainstones contain cross-beddingand ripple cross-lamination (Figure 8E), giant ooids(Figure 9A), composite coated grains and coatedintraclasts ranging up to 15 cm (6 in.) (Figure 9B,C), and peculiar isopachous bladed marine ce-ments that are interpreted to have encrusted sta-bilized surfaces within oolite shoals (Figure 9D).Significantly, in all areas where oolite is preservedat the platform or basin margin, there is a conspic-uous abundance of giant ooids typically ranging upto 5 to 10 mm in diameter (Figure 9). Such giantooids were found at margin localities of Bangeng,

Louhua, Dajing-Xiaojing, Guandao, Hanlong, andXiliang, circumscribing the GBG (Figure 2).

Oolites occur within the inner ramp (YelangFormation) and platform margin and interior (An-shun and Yongningzhen formations) of the LowerTriassic Induan andOlenekian strata of the Yangtzeplatform (Enos et al., 2006). During the Induan,the Yangtze platform developed a ramp profilewith the distal ramp characterized by laminated,dark, pyritic lime mudstone and shale of the Luo-lou Formation (Figures 10, 11). Middle-ramp fa-cies are characterzed by the Daye Formation, which

Figure 8. Macroscopic characteristics of oolites from the Great Bank of Guizhou (GBG). (A) Outcrop photograph of dolomitized oolitewith herringbone cross-beds in the lower part of the platform interior dolo-oolite at the Dajiang section. Reversing current directionsindicating tidal currents. The scale bar on the lower left is 10 cm (4 in.). (B) Oolite grainstone with ooids ranging up to 5 mm and roundedcoated intraclasts up to 1 cm (0.4 in.). From the interior of the GBG, proximal to the southern margin, south of Dawen (Figure 2 forlocation). (C) Supratidal fenestral laminite facies containing ooids and rounded oolite intraclasts (black arrows). Note fenestral pores,dissolved ooids with dropped nuclei (white arrows), and well-preserved ooids truncated at margins of intraclast. From the upper part ofdolo-oolite unit in platform interior at Dajiang (Figures 2, 7 for location). (D) Oolite fabric expressed on weathered surface of coarselycrystalline dolomite, ramp crest shoals on the northern margin of the GBG at Xiliang (Figure 2 for location). (E) Outcrop photograph ofripple forms in cross section in oolite from shoals on the southern margin at Louhua (Figure 2 for location).

Lehrmann et al. 1399

is composed dominantly of thin-bedded burrowedlime mudstone. The Daye and Luolou formationscontain interspersed carbonate breccias, which aredominantly composed of limemudstone clasts butinclude oolite clasts representing debris-flow de-posits shed basinward from shallower parts of theramp to the north and northwest (Figure 11). Up-dip, the Yelang Formation contains five to sixthick cliff-forming sequences of oolite interbed-ded with recessive mudrock and lime mudstone(Figures 11, 12A). Mudrock and sandstone pre-

1400 Geologic Note

dominate westward as the Yelang Formationchanges facies to the Feixianguan Formation to-ward the Khamdian massif (Enos et al., 2006).

The large-scale depositional sequences of theYelang Formation range from 50 to 200 m (164–656 ft) in thickness (Figure 11). The oolite-bearinglimestones that dominate the upper part of thedepositional sequences are amalgamated, cross-bedded, herringbone cross-bedded, and rippledoolite (Figure 12B) that shoal upward to limewackestones, lime mudstones, and flaser-bedded

Figure 9. Macroscopic characteristics of oolites with giant ooids from margin shoals on the Great Bank of Guizhou (GBG). (A) Giantooids in grainstone range up to 5 mm in diameter. From the southern margin of the GBG at Dajing-Xiaojing (Figure 2 for location). (B)Giant ooids up to 1 cm (0.4 in.) and coated intraclasts (composite coated grains) up to 7 cm (2.7 in.) across from the southern margin ofthe GBG at Bangeng (Figure 2 for location). The pen for scale is 16 cm (6.3 in.). (C) Oolite with giant ooids and enormous coatedintraclasts up to 15 cm (6 in.) in diameter. From the southern margin of the GBG at Louhua (Figure 2 for location). The hammer for scale is28 cm (11 in.) long. (D) Unusual isopachous bladed cement crusts, interpreted to be marine cement, encrusted onto a stabilized surface inoolite. Note the botryoidal and isopachous character of the cement. The photo is from the southern margin of the GBG at Louhua (Figure 2for location).

argillaceous mudstone representing tidal flats. Al-though the oolites are generally finer grained thanthose of the isolated platforms (average 0.5 mm,generally <1 mm), they also contain rare roundedintraclasts and composite coated grains up to 1 cm(0.4 in.) (Figure 12C). At Longchan, the YelangFormation is 773 m (2535 ft) thick and contains14% oolite (Figure 11).

During the Olenekian, the Yangtze aggradedto develop a platform profile with oolite shoals atthe margin and an interior low-energy lagoon withelevated salinity (Enos et al., 2006). The AnshunFormation is interpreted to represent platformmargin oolite shoals and back shoal tidal flats facingan interior subtidal lagoon represented by theYongningzhen Formation (Enos et al., 2006). TheAnshun Formation is predominantly coarsely crys-talline dolomite with relatively poor fabric preser-vation, although it contains ooids, sheet cracks, andtepee structures recognizable on weathered surfaces(Figure 11). Oolitic dolostone in the Zhenfeng area(Figure 2) is interpreted to represent margin shoalsand is predominantly massive noncyclic oolite, withooids typically ranging from 1 to 2 mm. Severalbeds contain giant ooids and composite coated grainsup to 2.5 cm (1 in.) in diameter (Figure 12D).

The cyclic facies of the Anshun Formation iscomposed of numerous meter-scale cycles shal-lowing upward from rippled, cross-laminated, andbioturbated subtidal oolitic, peloidal, intraclastic,and skeletal grainstones-packstones to supratidalfenestral laminated dolomudstone caps. Mud cracksand tepee structures also occur in supratidal facies,indicating subaerial exposure. Ooids within thesubtidal part of cycles are typically smaller thanthose found in margin shoals, 0.5 to 1 mm in di-ameter. Skeletal constituents consist of a restrictedfauna dominated by gastropods and bivalves. Majorflooding intervals also include open-marine con-stituents such as echinoderms and ammonoids.Regionally correlative dolobreccia intervals in theAnshun and Yongningzhen formations are inter-preted to be solution collapse breccias that marksequence boundaries (Enos et al., 2006). The cyclicfacies of the Anshun Formation is interpreted torepresent tidal-flat deposition in relatively low-energy environments protected by margin ooliticshoals and facing the low-energy lagoon representedby the Yongningzhen Formation (Figures 10, 11).The Anshun Formation ranges from 280 to 712 m(918–2335 ft) thick (Guizhou Bureau of GeologyandMineralResources, 1987). Because of the extensive

Figure 10. Reconstruction of the architecture of the Yangtze platform margin at Hongyan (Figure 2 for location). Note the Induan rampand Olenekian platform architecture of the Yangtze platform containing oolites in the Yelang and Anshun formations. Oolites also occurwithin clasts in subaqueous debris flow breccias in the Loulou Formation. Modified from Minzoni (2007).

Lehrmann et al. 1401

dolomitization, however, it is impossible to de-termine the proportion of oolite in various facieswithin the formation.

PETROGRAPHY

Oolites of the Nanpanjiang Basin have especiallywell-preserved fabrics in the interior and margin fa-

1402 Geologic Note

cies of the isolatedplatforms inGuangxi (Chongzuo-Pingguo, Heshan, and Debao platforms), wherethey are preserved as limestone in the Majiaolingand Beisi formations. Within the Great Bank ofGuizhou, the microcrystalline upper part of theinterior dolo-oolite and large areas of the interiorand margin escaped dolomitization and have ex-cellent fabric preservation. Within the Yangtzeplatform, the Yelang Formation contains oolites

Figure 11. Regional stratigraphic cross section of the Lower Triassic facies across the Yangtze platform interior (at Yongningzhen onthe right) to the basin margin (at Nilo, Zhenfeng on the left). The Induan Yelang Formation contains oolites developed in an inner-rampposition, whereas the Olenekian contains oolites developed at a platform-margin position (Lc section). See text for details. Modifiedfrom Enos et al. (2006); used with permission from the Geological Society of America. GBG = Great Bank of Guizhou; Nl = Niluo,Zhenfeng; Xm = Xinmin-Ziyun; Hy = Hongyan Guanling; Pzc = Pazhichang, Guanling; Lc = Longchan, Zhenfeng; Yn = Yongningzhen,Guanling; and SB = sequence boundary.

preserved as limestone, and although the AnshunFormation is coarsely crystalline dolostone, lo-cal areas are preserved as limestone with originalfabrics.

From consideration of field and petrographicobservations, the oolites of the Nanpanjiang Basincan be subdivided broadly into platform interiorand margin shoal facies.

Platform-interior oolite facies are predominant-ly grainstone with ooids ranging from 0.1 to 1.5mmin diameter, typically averaging about 0.5 mm(Figure 13A). Some interior oolite units are grain-stone with distinctly bimodal grain-size distribu-tion (Figure 13B). Ooids are approximately 0.5 mmin diameter; the smaller mode is composed pre-dominantly of spherical micritic peloids and smallrounded skeletal fragments about 0.2 mm in di-ameter (Figure 13B). Ooid nuclei are rarely ob-served in random cuts but include rounded mi-critic grains, rounded fragments of other ooids, androunded skeletal fragments (Figure 13C, D). Thecoarse neomorphic spar of skeletal nuclei indicates

that they are molluskan fragments. Ooid fragmentnuclei were commonly well rounded before re-coating by cortical laminae (Figure 13B).

The inner parts of cortices commonly have aradial fabric with a well-preserved fabric detail,indicating a probable original calcitic composition(cf. Sandberg, 1985; Tucker, 1985) (Figure 13D).Outer cortices are composed of fine concentricmicritic laminae, locally alternating with radialcortices (Figure 13D, E) but more commonly al-ternating with layers of coarsely crystalline mosaicof calcite spar (Figure 13F). The detailed fabricpreservation of the micritic layers and the coarsesparry replacement of other cortex layers suggestthey were originally composed of calcite and ara-gonite, respectively. Thus, we infer that the ooidswere originally bimineralic with well-preservedradial and micritic cortical laminae that were highMg calcite, alternating with layers in the outercortex composed of coarse spar that were originallyaragonite (cf. Sandberg, 1985; Wilkinson et al.,1985; Chow and James, 1987).

Figure 12. Macroscopic char-acteristics of oolite facies in theYangtze platform. (A) Panoramic out-crop photograph of oolite-bearingcarbonate-siliciclastic sequences of theYelang Formation at Yongningzhen(Figure 2 for location). Cliff-formingridges are the upper regressive partof depositional sequences bearingooid lime grainstones and packstones,lime mudstone, and argillaceousflaser-bedded ribbon rocks. Reces-sive parts are transgressive shale-dominated components of sequences.Two large cliff-forming oolite unitsare seen in the foreground, a thirdis visible as a ridge on the skyline.(B) Cross-bedding in the oolite of theYelang Formation at Yongningzhen.(C) Oolite with ooids ranging up to6 mm. Note the bimodal grain size.From Yongningzhen Formation, 188m(617 ft) above the base of the forma-tion (Figure 11 for location). (D) Oolitewith ooids up to 5 mm and compositecoated grains up to 2.5 cm (1 in.) in

diameter. From Anshun Formation,Zhenfeng (Figure 2 for location).

Lehrmann et al. 1403

Some ooids show compaction through col-lapsed or distorted cortices. Selective dissolutionof cortical layers resulted in the brittle collapseand compaction of micritic cortical layers. Such“eggshell” compaction supports the interpretationof an original aragonite component that has beendissolved out, causing the collapse of unsupportedouter cortical layers during early burial diagenesis(cf. Wilkinson and Landing, 1978; Wilkinson et al.,1983). This phenomenon is especially common ingiant ooids from margin shoal environments, asdiscussed in detail below.

1404 Geologic Note

Constituents in platform interior oolites in-clude rare giant ooids greater than 2 mm, compo-site coated grains, intraclasts, and skeletal fragments(Figure 8B). Giant ooids are much less commonthan in margin facies. Composite coated grains areessentially oolite clasts in which ooids were ce-mented together as incipient intraclasts or grape-stone aggregates and then rounded and recoated(Figure 13F). The largest ooids, composite coatedgrains, and intraclasts were likely transported frommargin shoals to the platform interior duringstorms.

Figure 13. Petrographic characteristics of platform interior oolites (Figure 2 for locations). (A) Ooid lime grainstone from the interior ofthe Chongzuo-Pingguo platform, Taiping section, 90 m (295 ft) (Figures 2, 4 for location). (B) Bimodal oolite, larger ooids with nucleicomposed of rounded fragments of preexisting ooids (upper left). Note also abraded ooid (lower right), Chongzuo-Pingguo platform,interior near the northern margin. Ooids range up to 1.2 mm in diameter, distinctly smaller mode of very fine sand size are roundedmicrite grains (peloids) and superficial ooids. (C) Ooid lime grainstone with elongate ooids containing bivalve fragment nuclei. GreatBank of Guizhou (GBG), Dawen section, 90 m (295 ft). (D) Ooids with radial inner cortical fabrics (upper right) and outer cortical fabricsconsisting of thin well-preserved micritic layers. Note that a few ooids contain a combination of micritic and radial fabrics in the outercortex (arrows). From carbonate turbidite beds in the outer ramp on the northern margin of the GBG at Hanlong. (E) Ooids with radialinner cortical fabrics (upper right) and outer cortical fabrics consisting of thin well-preserved micritic layers. From the interior of theChongzuo-Pingguo platform, at Taiping section, 390 m (1246 ft). (F) Large ooids exceeding 4 mm, with nuclei composed of rounded ooidand composite coated grain fragments. From the northern margin of the Chongzuo-Pingguo platform.

Dolo-oolite in the interior of the GBG in-cludes a supratidal microbial laminite facies thatprograded over stabilized oolite shoals. The supra-tidal facies contains ooids and rounded oolite grain-stone clasts trapped between fenestral microbiallayers (Figure 8C). The ooids are typically dis-solved, producing molds and ooids with droppednuclei. The ooids were likely transported from ad-jacent shoals to the tidal flat during storms and ex-perienced dissolution during subaerial exposure.Additional evidence for exposure includes meniscuscements. Intraclasts were cemented and roundedbefore transport to the tidal flat and contain ex-quisitely preserved cortical fabrics, including theradial, micritic, and coarse spar layers previouslydiscussed (Figure 13F). Diagenesis includes a fringeof early marine isopachous bladed cement (colum-nar spar) probably originally precipitated as a highMg calcite, followed by dissolution of ooid corticallayers and the collapse of ooids, precipitation ofequant spar-filling intergranular space and molds,and finally dolomitization. Coarse dolomite replace-ment (rhombs up to 1 mm) completely obliteratedfabrics in many cases or preserved ooids as faintghost fabrics. Notably, dolomitization produced sig-nificant intercrystalline porosity, which was chargedwith hydrocarbons, leaving bitumen stains in manyplatform interior samples.

For the most part, margin oolites are similar tothose just described for platform interiors, exceptfor evidence of higher energy conditions, such ascross-bedding, and coarser grain size with abun-dant oversized ooids greater than 2 mm in diam-eter. Giant individual (noncomposite) ooids, com-monly about 2.5 to 5 mm in diameter, but locallyreaching 1 cm (0.4 in.) in diameter and compositecoated grains typically about 3 cm (1.2 in.) in di-ameter (Figures 9, 14) were found in grainstonesfrom bank-margin facies on the northern marginof the CPP, the southern and northern margins ofthe GBG, and the margin of the Yangtze platform(Figure 2). Thus, these coarser grained oolites arewidespread in platform-margin facies. The nucleiof these giant ooids are dominantly rounded frag-ments derived from preexisting ooids (Figures 13F,14C), although rounded micrite nuclei (peloids)are also common. In some cases, elongate spalled

cortical fragments comprise the nucleus. Intra-clasts may also have cortical coatings and showevidence of repeated episodes of abrasion andcortical recoating, which smooths out shape ir-regularities and leads to extremely well-roundedgrains (Figure 14C, D).

Like the platform interior oolites, cortical fab-rics of margin oolites contain micritic layers withfine fabric preservation alternating with coarsesparry cortical layers that are either sparry fill oflayers that were dissolved or are a coarse neo-morphic replacement spar that does not preserveoriginal textures (Figures 14C, D; 15A). Locally,the outer cortical layers also contain brickworkfabrics indicative of original aragonite mineral-ogy (Figure 15B). The cortical fabrics suggest thatoriginal mineralogy was bimineralic with the radialand micritic layers originally composed of highMg calcite, whereas the dissolved or neomorphosedcoarse sparry layers were originally aragonite. Al-though we have not done geochemical analysis toverify the original mineralogy (elevated Mg or Srlevels indicative of a high Mg calcite or aragonite,respectively), numerous studies have reported bi-mineralic ooids with fabrics identical with thosedescribed herein, and those studies have revealedgeochemistry consistent with our mineralogic in-terpretation (Wilkinson et al., 1983; Tucker, 1984;Chow and James, 1987;Major et al., 1988; Chatalov,2005). Notably, in each of the aforementionedstudies, the radial cortical fabrics, interpreted tohave been highMg calcite, are concentrated in theinner part of the ooids and concentric or sparrylayers interpreted to have been aragonite occur inouter cortices. In many cases, the outer micriticcortical layers are collapsed by brittle compactionas aragonitic cortical layers were dissolved duringearly burial diagenesis (Figure 15C–E). Brittle col-lapse and spalling of cortical laminae in these ooidsclearly occurred during early burial (not during de-position, as a few authors have suggested, e.g.,Carozzi, 1961; Sarkar, 1983) because adjacent grainshave been compressed into the ooid at the pointof collapse (Figure 15E). Furthermore, isopachousbladed cement crusts on ooids, likely precipitated bymarine phreatic fluids during early burial, also ex-perienced brittle deformation during ooid collapse

Lehrmann et al. 1405

(Figure 15D, E), demonstrating postdepositionaldistortion. Such brittle eggshell collapse features,formed where cortical layers were selectivelydissolved, have been widely interpreted to rep-resent original aragonite composition (Wilkinsonand Landing, 1978; Wilkinson et al., 1983; Chowand James, 1987; Zempolich et al., 1988; Chatalov,2005).

The large grain size, rounded fragments of shat-tered ooids, large rounded intraclasts, and evidenceof multiple surfaces of abrasion and recoating ofooids indicate a high-energy current agitation inmargin oolite shoals. Possibly, the ooids grew up to

1406 Geologic Note

the maximum size for grain growth by accretionunder the prevailing energy regime and began tosuffer abrasion (cf. Heller et al., 1980; Swett andKnoll, 1989; Sumner and Grotzinger, 1993). Thegreater abundance of giant ooids in bank-marginenvironments indicates a high wave and tidal cur-rent energy on margin shoals.

DISCUSSION

Lower Triassic strata contain numerous unusualfacies and diagenetic features that have been

Figure 14. Petrographic characteristics of platform-margin oolite grainstone facies. (A, B) Ooid lime grainstone with giant ooids fromthe southern margin of the Great Bank of Guizhou at Bangeng (Figure 2 for location). (A) Contains ooids up to 1 cm (0.4 in.) andcomposite coated grains more than 2 cm (0.8 in.) across. (B) Contains ooids with consistent size 7 to 8 mm in diameter. (C) Ooliticgrainstone with ooids up to 3 mm and composite coated grains from the northern margin of the Great Bank of Guizhou, Xiliang area.Note the alternation of micritic cortical layers with a detailed fabric preservation and sparry layers with destroyed fabrics and brittlecompaction of adjacent layers (arrows). Sparry layers are interpreted to have originally been aragonite. (D) Oolitic lime grainstone withrounded intraclasts and composite coated grains showing an evidence of repeated abrasion and recoating. Arrows indicate alternation ofcortices with fabric-destructive sparry layers (original aragonite) and micrite layers (calcite). Southern margin of the Great Bank ofGuizhou, Bangeng area.

attributed to low-biodiversity and anomalous envi-ronmental conditions in the aftermath of the end-Permianmass extinction (Sano andNakashima, 1997;Kershaw et al., 1999; Lehrmann, 1999; Twitchett,1999; Woods et al., 1999; Lehrmann et al., 2001,2003; Pruss and Bottjer, 2004; Pruss et al., 2005;Payne et al., 2006a, 2007; Wang et al., 2005). The

end-Permian mass extinction resulted in a reduc-tion in marine biodiversity back to Cambrianlevels with the loss of 79% of marine animal genera(Payne and Clapham, 2012). Anomalous faciesattributes include widespread open-marine micro-bialites reported globally from the paleotropics(cf. Payne et al., 2007), low levels of bioturbation

Figure 15. Cortical fabrics and compaction fabrics in platform-margin oolites. (A) Cortical fabrics showing alternating micritic layerswith a detailed fabric preservation alternating with coarse sparry calcite layers with partially destroyed fabric. Layers are interpreted tohave originally been high Mg calcite and aragonite, respectively. From clast in subaqueous debris-flow breccias in the Daye Formation,Yangtze platform, Gaimao section, Guiyang area (Figure 2 for location). (B–E) From the northern margin of Great Bank of Guizhou atGuandao section (Figure 2 for location). (B) Ooid with micritic cortical laminae alternating with somewhat coarser layers with thebrickwork structure interpreted to result from recrystallization of tangential aragonite. The fabric is especially well represented in theouter cortex (rectangle). (C) Polished slab photograph showing compaction fabrics in giant ooids. Micritic cortical layers are white,whereas sparry layers are dark. Note that ooids that experienced brittle deformation of micritic layers collapsed around sparry layers(black arrows). Ooids with minimal compaction are predominantly micritic (lower left). In the center, slab ooids have been completelycompacted and are nearly unrecognizable (white arrow). (D) Compacted ooids. The broken outer cortical layer in ooid on the upperright has the fringe of isopachous cement that was also truncated during compaction (arrows), demonstrating postdepositional dis-tortion. The large volume of ooid on the lower left is missing apparently because of dissolution of aragonitic part of cortex. The remnantof the outer micritic cortical layer is barely intact. (E) Two ooids compacted into one another. Brittle compaction of micritic, calciticcortical layers around loss of volume from missing aragonitic areas. Note the outermost cortical layer with fragmented isopachouscement fringe (arrows).

Lehrmann et al. 1407

and flaser-bedded intertidal ribbon rocks (Twitchett,1999; Lehrmann et al., 2001), and sea-floor cementfans (Woods et al., 1999). Oolites have been re-ported from Lower Triassic carbonate ramp com-plexes in numerous localities (Baud et al., 2005;Chatalov, 2005; Aljinovic et al., 2006; Farabegoliet al., 2007; Haas et al., 2007). Groves and Calner(2004) suggested that oolites deposited in the im-mediate aftermath of the end-Permian extinctionon carbonate platforms in Turkey represented “di-saster oolites” or a default mode of sedimentationin the absence of skeletal biota following the end-Permian mass extinction. This study reveals that(1) oolites are extremely widespread among LowerTriassic carbonate platforms in the NanpanjiangBasin as margin shoals and amalgamated prograd-ing interior shoal complexes; (2) ooids of the EarlyTriassic were bimineralic, composed of aragoniteand high Mg calcite; and (3) giant or oversizedooids more than 2 mm were prevalent in marginshoal complexes.

Oversized or giant ooids more than 2mmhavebeen reported primarily from Precambrian strata(Belt Group, Montana, Tucker, 1984; Biri Forma-tion, Norway, Tucker, 1985; Eleonore Bay Group,Greenland, and Akademikerbreen Group, Spits-bergen, Swett and Knoll, 1989; Beck Springs Do-lomite, California, Zempolich et al., 1988; GhaapGroup, South Africa,Wright and Altermann, 2000;and Deoban Limestone, India, Srivastava, 2006).The few reports of oversized ooids of Phanerozoicage include the Cambrian (Port au Port Group,Newfoundland, Chow and James, 1987), the LowerTriassic (Great Bank of Guizhou, south China,Payne et al., 2006b; Li et al., in press; UntereBuntsandstein Group, Germany, Weidlich, 2007),and the Jurassic (Tithonian of Croatia, Husinec andRead, 2006). The oversized ooids from the Jurassicare interpreted to have formed in quiet hypersalinewaters during transgression and flooding of a car-bonate platform (Husinec and Read, 2006).

Numerical modeling shows that several fac-tors can increase ooid size, including a low supply ofnuclei, a high accretion rate, or high current ve-locities (Sumner and Grotzinger, 1993). Some au-thors have concluded that oversized ooids prevailedduring the Precambrian because a high calcium

1408 Geologic Note

carbonate saturation in seawater forced abioticand microbial precipitates as the primary carbonateprecipitation sink in the absence of calcifying skel-etal organisms (cf. Swett and Knoll, 1989; Sumnerand Grotzinger, 1993). Sumner and Grotzinger(1993) inferred that a high seawater saturation,high accretion rates, and a low nucleus supplyalone could not explain the occurrence of giantooids (>1 cm) in the Neoproterozoic strata becausesuch conditions should have existed throughoutthe Precambrian, whereas Archean and Meso-proterozoic ooids, while oversized, are generallysmaller (<5 mm) than the Neoproterozoic ones.They concluded that a predominance of ramp-stylearchitectures and high-energy conditions helpedaccount for the extreme size of theNeoproterozoicooids (Sumner and Grotzinger, 1993).

Lower Triassic oolites of the Nanpanjiang Basinsuggest a similar set of controls. The prevalence ofgiant ooids and large composite coated grains inbank-margin shoal settings and shattered ooids andabrasion with recoating fabrics in larger grains sup-ports high current velocities as a requisite condi-tion for the generation of these giant ooids. How-ever, althoughmany other high-energy oolitic rampsystems existed during the Phanerozoic, giant ooidsare rare. During the Early Triassic, like the Pre-cambrian, high-energy conditions coupled with ahigh seawater calcium carbonate concentration in aperiod of reduced skeletal carbonate precipitationlikely explain the widespread occurrence of giantooids.

This scenario, however, does not explain thewidespread presence of giant ooids in the Neo-proterozoic and the Lower Triassic, in contrast tosmaller sizes typical of the Archean and Mesopro-terozoic (<5 mm), which have a similar absence ofskeletal sinks for calcium carbonate. We hypothe-size that the explanation lies in the influence ofocean redox chemistry on the spatial distribution ofcalcium carbonate saturation state. Higgins et al.(2009) showed that the gradient in calcium car-bonate saturation from surface to deep water wouldbe greatly reduced in anoxic oceans because of thecontrasting effects of aerobic versus anaerobic res-piration pathways on seawater alkalinity. Anoxicoceans and the resulting dominance of anaerobic

respiration lead to CaCO3 saturation in shallowwater that is reduced in comparison with an oxicocean and deep-water saturation that is elevated incomparison with an oxic ocean because the manypathways for anaerobic respiration (e.g., sulfatereduction, iron reduction) produce alkalinity, un-like aerobic respiration.

Although ocean anoxia was prevalent duringthe Neoproterozoic and Early Triassic, atmosphericoxygen levels were undoubtedly higher than thosethat characterized the Archean through Mesopro-terozoic, and consequently, rates of aerobic respi-rationwere likely higher as well. Consequently, wehypothesize that although ocean anoxia was wide-spread, the Early Triassic (and Neoproterozoic)CaCO3 saturation gradient with depth remainedmuch steeper than that typical of the Mesopro-terozoic and earlier times. A low skeletal carbonateproduction and a high carbonate substantial sat-uration state in shallow water combined to favorthe rapid growth of very large ooids. In short, theEarly Triassic loss of skeletal carbonate produc-tion was likely more important than the reducedCaCO3 saturation gradient caused by ocean an-oxia. This scenario can also explain why evidencefor in-situ abiotic precipitation of calcium carbon-ate in outer shelf to deep-basinal environments,while present (Woods et al., 1999), is relativelysparse in the Lower Triassic, instead of a pervasivefeature in the Lower Triassic basin margins. LowerTriassic slope and basin margin facies in the Nan-panjiang Basin are dominated by sediment re-deposited from shallow-water environments (e.g.,Lehrmann et al., 1998; Enos et al., 2006) instead ofby sea-floor crystal fans or micritic crusts.

Fluctuations in seawater chemistry from ara-gonite seas (Neoproterozoic–Middle Cambrian,Middle Mississipian to Early Jurassic, and Paleo-gene to Holocene) to calcite seas (Middle Cambrian–Middle Missisippian; Late Jurassic–Paleogene) re-flected in the secular variation in mineralogy ofmarine cement and ooids have been variously in-terpreted to result from fluctuations in pCO2, Ca/Mg ratio, or SO2�

4 dissolved in seawater (Sandberg,1983; Wilkinson et al., 1985; Wilkinson and Given,1986; Hardie, 1996; Bots et al., 2011). Aragonite-sea systems contain both aragonite and high–Mg

calcite ooids and cements, including bimineralicooids, whereas systems with calcite seas containexclusively calcitic ooids and cements (Wilkinsonet al., 1985; Hardie, 1996). The occurrence of bi-mineralic ooids (reflecting aragonite seas) in theEarly Triassic argues against pCO2 as a major fac-tor driving the long-term secular switching betweenaragonite and calcite seas. The Early Triassic was atime of extreme global warming at the beginningof theMesozoic greenhouse episode and probablyhad extremely high CO2 levels associated withSiberian traps volcanism and/or methane releaseassociated with the end-Permian mass extinction(cf. Read, 1998; Retallack 1999; Krull et al., 2004;Payne et al., 2007; among many others). If pCO2

were the primary driver switching aragonite seasto calcite, then the switch to the Mesozoic calcitesea should have occurred by the Early Triassicinstead of being delayed until the Late Jurassic.Furthermore, Luo et al. (2010) argued for ex-tremely low SO2�

4 concentrations during the EarlyTriassic, which should have encouraged calcite seaconditions (Bots et al., 2011). A gradual increase inCa/Mg concentration associated with a change inhydrothermal alteration at sea-floor spreading cen-ters is therefore left as the most likely mechanismfor the switch from aragonite sea to the calcite seain the Late Jurassic.

IMPLICATIONS FOR HYDROCARBONEXPLORATION AND DEVELOPMENT

Dolomitized oolites in the platform interior faciesof the GBG and the CPP of the Nanpanjiang Basincontain bitumen stains in intercrystalline porosity,indicating that hydrocarbons once migrated intothese platforms. Significant hydrocarbon reservoirsoccur in oolite-bearing Lower Triassic carbonateplatform strata in the Feixianguan Formation of theSichuan Basin and in the Khuff and Kangan for-mations of the Middle East. Observations reportedherein of the architecture, size distribution, min-eralogy, and diagenesis of Nanpanjiang Basin oo-lites have important implications for hydrocarbonexploration and development in Sichuan and theMiddle East.

Lehrmann et al. 1409

The closest analog is the Feixianguan Forma-tion of the Sichuan Basin, which occurs within theSouth China block to the northwest of the Nan-panjiang Basin (Enos, 1995). Oolites of the Feix-ianguan Formation have been reported as a primaryreservoir facies in the Sichuan Basin. Similar to theoutcrops described from the Nanpanjiang Basin,the Feixianguan oolites have been interpreted asrepresenting platform or ramp margin shoals thatextend into platform interiors, thinning away fromthe margin (Ma et al., 2006, 2007; Peng et al.,2010). Although we have not found reference togiant ooids in the Sichuan Basin, petrographic fab-rics such as dissolved and collapsed ooids, brittleeggshell compaction of micritic cortices, and alter-nating cortical laminae of well-preserved micriteand coarse neomorphic spar (figured in Ma et al.,2006; Tan et al., 2011) are identical with those de-scribed herein for the Nanpanjiang Basin. Oolitereservoirs in the Sichuan Basin are mostly dolomi-tized, with much of the reservoir potential fromsecondary moldic, intercrystalline, vuggy, and frac-ture porosity, although significant interparticle po-rosity is also important (Ma et al., 2006; Pan et al.,2010; Peng et al., 2010; Tan et al., 2011).

Reservoirs in the Khuff and Kangan formationsalso contain oolites described as shoal facies andtransgressive to highstand facies stacked in depo-sitional cycles developed across a vast homoclinalramp (Alsharhan, 2006; Insalaco et al., 2006;Esrafili-Dizaji and Rahimpour-Bonab, 2009; Peyraviand Kamali, 2010). The basal Triassic immediatelyabove the Permian–Triassic boundary contains acoarse-grained pebbly grainstone to packstonecontaining ooids, composite coated grains, oncoids,and rounded intraclasts (Insalaco et al., 2006;Ehrenberg et al., 2008). This coarse-grained fa-cies contains giant ooids similar to those describedfrom the Nanpanjiang Basin. Overlying cycles con-tain reservoirs with oolite dolostones. Porosity ispredominantly secondary, moldic, and intercrystal-line (Holail et al., 2006; Ehrenberg et al., 2007).In their comparison between reservoir facies ofthe Triassic Khuff and Jurassic Arab formations,Ehrenberg et al. (2007) suggested that the TriassicKhuff ooids most likely had an original aragonitecomposition leading to porosity inversion, whereas

1410 Geologic Note

the Arab ooids had an original calcitic mineralogyresulting in lesser diagenetic alteration and greaterpreservation of interparticle porosity. Further in-vestigation is needed to evaluate whether theooids of the Khuff were originally bimineralic andhow this would impact reservoir quality.

There have not been reports of isopachous ma-rine cements from the Feixianguan or Khuff for-mations that are comparable to those observed inthe platform margin oolites of the Nanpanjiang Ba-sin. If similar cements are found in the Feixianguanor Khuff formations, they should be expected tohave a significant effect on reservoir partitioning.

CONCLUSIONS

1. Oolites are widespread in the Lower Triassicisolated carbonate platforms of theNanpanjiangBasin and in the attached Yangtze platform thatborders the basin. The oolites formed high-energyshoal complexes at ramp and platform marginsand form sheetlike deposits in several large-scaleshallowing-upward depositional sequences thatcorrelate across platform interiors. Amalgamatedoolite grainstone beds in platform interiors reach50 m (164 ft) in thickness. The areal distribu-tion of oolites in isolated platforms may exceed10,000 km2 (3844 mi2) and is probably evengreater in the Yangtze platform.

2. Oolite grainstone units thicken and becomecoarser grained toward bank margins, indicat-ing the development of high-energy shoals atthe margin. Margin shoals contain oversized(giant) ooids commonly 5 to 7mm in diameterbut occasionally reaching up to 1 cm (0.4 in.)in diameter and composite coated grains up to15 cm (6 in.) across. Fragmented ooids and re-peated abrasion and recoating of larger coatedgrains demonstrate high-energy conditions inplatform-margin shoals. Energy levels may havereached the upper limit of ooid size where ac-cretion gives way to abrasion. Interior oolites arealso grainstones but with finer grain size, typi-cally about 0.7 mm. Rare giant ooids and largecomposite coated grains in interior sections

were probably transported to the interior dur-ing storms.

3. Similar to Precambrian systems, a scenario ofhigh calcium carbonate saturation and high-energy conditions may explain the prevalenceof giant ooids in margin shoal environments inthe Lower Triassic platforms. A high calciumcarbonate saturation in seawater would be ex-pected because of reduced skeletal precipitationin the aftermath of the end-Permian mass ex-tinction. The preferential development of giantooids and fragmented and abraded ooids alsosuggests that high-energy conditions were im-portant factors in the genesis of large ooids. Thepresence of similarly giant ooids on carbonateplatforms of Neoproterozoic age but their ab-sence on older Precambrian platforms may re-sult from the increased gradient in calcium car-bonate saturation state with depth because ofthe increased importance of aerobic respirationin the oceans as atmospheric pO2 increased.Despite the widespread evidence for the EarlyTriassic ocean anoxia, such conditions did notrepresent a full return to Archean or Paleopro-terozoic conditions caused by the persistence ofhigh levels of oxygen in the atmosphere.

4. Petrographic observations indicate that the ooidstypically have nuclei composed of rounded mi-crite or rounded fragments of broken ooids. In-ner cortical laminae and some outer cortical lam-inae contain well-preserved radial fabrics. Outercortical laminae consist of well-preserved micriticlayers alternating with layers of coarse neomor-phic spar or coarse void-filling spar. Comparisonwith other ooids described in the literatureindicates that these were bimineralic ooids inwhich the radial and micritic layers were orig-inally composed of high Mg calcite and thecoarse neomorphic and void fill layers were orig-inally aragonite. Ooids with cortical layers thatexperienced dissolution are commonly com-pacted with brittle eggshell collapse of micriticcortical layers.

5. The occurrence of bimineralic aragonite and highMg calcite ooids in the Lower Triassic is compat-ible with the pattern of secular variation in sea-water chemistry (aragonite-calcite seas) that has

been recognized in numerous studies. The de-velopment of aragonite-bearing ooids during theEarly Triassic, a time of rapidly increasing atmo-spheric ocean CO2 at the beginning of the Me-sozoic greenhouse and a time of extreme CO2

and low SO2�4 associated with the end-Permian

mass extinction, argues against changes in CO2

or SO2�4 concentrations as the major driving

mechanisms causing the secular aragonite-calcite variations in seawater chemistry. Thechange in Ca/Mg ratio is therefore preferred asthe mechanism driving the Late Jurassic shiftfrom aragonite to calcite seas.

6. The architecture and petrographic characteris-tics of the Lower Triassic oolites of the carbon-ate platforms of the Nanpanjiang Basin presenta useful analog for better understanding age-equivalent hydrocarbon reservoirs in the Si-chuan Basin of south China and the Khuff res-ervoirs of the Middle East.

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