Facies, stratigraphy, and evolution of a middle Ediacaran carbonate ramp Khufai Formation

Facies,stratigraphy,and evolution of a middle Ediacaran carbonate ramp:Khufai Formation,Sultanate of Oman Magdalena Osburn,John Grotzinger,and Kristin BergmannA B S T RA C TThe Khufai Formation is the oldest carbonate platform of the Cryogenian to lowermost Cambrian Huqf Supergroup.A strati-graphic characterization of this unit includes detailed facies descriptions,a sequence-stratigraphic interpretation,and evalu-ation of lateral heterogeneity and overall ramp evolution.The Khufai Formation comprises one and one-half depositional sequences with a maximum flooding interval near the base of the formation and a sequence boundary within the upper peritidal facies.Most of the deposition occurred during highstand progra-dation of a carbonate ramp.Facies tracts include outer-ramp and midramp mudstones and wackestones,ramp-crest grainstone shoal deposits,and extensive inner-ramp,microbially dominated peritidal deposits.Outcrops in the Oman Mountains are deep-water deposits,including turbiditic grainstone and wackestone interbedded with siliciclastic-rich siltstone and crinkly laminite. Facies patterns and parasequence composition are variable both laterally across the outcrop area and vertically through time because of a combination of ramp morphology,siliciclastic sup-ply,and possible syndepositional faulting.The lithostratigraphic boundary between the Khufai Formation and the overlying Shuram Formation is gradational and represents significant flood-ing of the carbonate platform.The stratigraphic characterization presented here along with the identification of key facies and dia-genetic features will help further future exploration and produc-tion of hydrocarbons from the Khufai Formation.Copyright©2014.The American Association of Petroleum Geologists.All rights reserved. Manuscript received January30,2013;provisional acceptance November27,2012;revised manuscript received January31,2013;final acceptance July29,2013.DOI:10.1306/07291312140A U T H O R SMagdalena Osburn∼California Institute of Technology,1200E.California Boulevard, Pasadena,California;maggie@earth. Magdalena Osburn is a geobiologist who uses sedimentology and organic geochemistry to understand the history of life on Earth and microbial influences on biogeochemical processes.She has a B.A. degree in earth and planetary sciences from Washington University in St.Louis and a Ph.D.in geobiology from the California Institute of Technology.John Grotzinger∼California Institute of Technology,1200E.California Boulevard, Pasadena,California;grotz@ John Grotzinger is a geologist interested in the evolution of surficial environments on Earth and Mars.Field mapping studies are the starting point for more topical laboratory-based studies involving geochemical,geological,and geochronological techniques.Kristin Bergmann∼California Institute of Technology,1200E.California Boulevard, Pasadena,California;bergmann@caltech. eduKristin Bergmann received degrees in geology from Carleton College(B.A.degree) and the California Institute of Technology (M.Sc.degree)where she is currently a Ph.D.candidate.She is studying the carbonate sedimentology,stratigraphy,and geochemistry of units within the Huqf Supergroup and is applying carbonate clumped isotope thermometry to problems of carbonate diagenesis and paleoclimate.A C K N O W L E D G E M E N T SWe thank William Wilks,Gordon Forbes, Gideon Lopes-Cardozo,Joachim Amthor, and other Petroleum Development Oman personnel for helpful discussions;Salim Al Maskery for field support and logistics;the participants of the2009Agouron Advanced Field Course for initial stratigraphic observation;the Ministry of Oil and Gas, Sultanate of Oman,for permission toAAPG Bulletin,v.98,no.8(August2014),pp.1631–16671631INTRODUCTIONNeoproterozoic strata of Oman provide an exceptionally complete and well-preserved sedimentary record of a highly variable period in Earth history.Recorded in these sediments are two of the pro-posed Snowball Earth glaciations,the largest carbon isotopic excursion in Earth history,the origination of Ediacaran calcified metazoans,and the Precambrian –Cambrian boundary (Burns and Matter,1993;Wood et al.,2002;Allen,2007;Bowring et al.,2007).In addition,the Huqf Supergroup hosts the oldest commercially viable hydrocarbon system with producing reser-voirs in uppermost Ediacaran to lowermost Cambrian rocks (Al-Siyabi,2005).Regional stratigraphic surveys of the Huqf Supergroup pro-vide a basic understanding of the depositional setting,and chemo-stratigraphic studies document concurrent changes in marine chemistry and enhance regional stratigraphic correlation (Wright et al.,1990;Burns and Matter,1993;McCarron,1999;Halverson,2005;Fike et al.,2006;Le Guerroue et al.,2006a;Fike and Grotzinger,2008).Considerable attention was devoted to understanding the older siliciclastic Masirah Bay Formation shelf sequence (Allen and Leather,2006),the overlying Shuram Formation (Le Guerroue et al.,2006a),and the younger Buah Formation carbonate ramp (Cozzi and Grotzinger,2004;Cozzi et al.,2004).Conversely,relatively little is known about Khufai Formation development,beyond its inclusion in regional surveys (Gorin et al.,1982;Wright et al.,1990;McCarron,1999).A firm understanding of sedimentological and stratigraphic context is necessary for evaluation of both geochemical measure-ments and the potential for hydrocarbon exploration.We have undertaken this study to better understand the processes,environ-ments,and time scale of Khufai Formation deposition.This article presents an analysis of the facies distributions from both platform and distal sections along with a detailed sequence-stratigraphic interpretation of the Khufai Formation.We evaluate the strati-graphic position and extent of depositional hiatuses in the upper Khufai Formation to better interpret chemostratigraphic records.The presentation of key facies and facies relationships in strati-graphic context will provide a valuable framework for future hydrocarbon exploration and development.Geological SettingThe Huqf Supergroup is a sequence of Cryogenian to Lower Cambrian sedimentary rocks,deposited on Archean to Neoproterozoic crystalline basement (Allen,2007;Bowring et al.,2007).Huqf strata outcrop in four main areas including theconduct and publish this research;and Michael Pope,Erwin Adams,and Stephan Schroder for helpful and constructive reviews.The AAPG Editor thanks the followingreviewers for their work on this paper:Erwin W.Adams,Michael Pope,and Stefan Schroeder.E D I T O R ’S NO T EColor versions of Figures 6–13can be seen in the online version of his paper.1632Stratigraphic Characterization of the Khufai Formation,OmanHuqf-Haushi uplift and Mirbat areas of central and southern Oman and near Jebel Akhdar and Saih Hatat in the Oman Mountains (Figure 1).The Huqf Supergroup comprises,in ascending order,the Abu Mahara,Nafun,and Ara Groups representing glacio-marine sediments,marine siliciclasticsandFigure 1.Regional map of Oman illustrating the outcrop localities for the Huqf Supergroup,regional tectonic features,and salt basins.Modified after Bowring et al.(2007).O SBURN ET AL .1633carbonates,and carbonates cyclically interstratified with sulfate and halite evaporites,respectively.The Nafun Group was further subdivided (Figure 2)into the successive Hadash,Masirah Bay,Khufai,Shuram,and Buah Formations (Gorin et al.,1982;Wright et al.,1990;Forbes et al.,2010).The Hadash Formation is a thin dolostone capping the Ghadir Manquil (Marinoan)glacial deposits,but is included in the Nafun Group.The overlying four units com-prise two large siliciclastic-to-carbonate cycles;where the Masirah Bay and Shuram Formations con-tain primarily siltstone and sandstone with minor car-bonate,and the Khufai and Buah Formations contain primarily carbonate facies with subordinate siliciclas-tic components (Forbes et al.,2010).The NafunGroup is well exposed on the Huqf-Haushi uplift (Huqf area)and in the wadis of Jebel Akhdar.Age Control and Tectonic SettingThe absolute ages of the Abu Mahara and Ara Groups are relatively well constrained by U-Pb dat-ing,but few chronostratigraphic constraints exist in the intervening Nafun Group.The Abu Mahara Group is deposited on approximately 824m.y.-old granodioritic basement (Bowring et al.,2007).A tuffaceous sandstone within the Ghubrah Member of the lower Abu Mahara Group was deposited at approximately 713Ma (Bowring et al.,2007),revi-sing upward a previous date of 723+16/-10MaFigure 2.Stratigraphic nomenclature for the Huqf Supergroup.Basic lithostratigraphic logs of the Nafun Group in the Huqf-Haushi high and Oman Mountain areas.Modified after Forbes et al.(2010),Bowring et al.(2007),McCarron (1999),and Allen and Leather (2006).1634Stratigraphic Characterization of the Khufai Formation,Oman(Brasier et al.,2000).A population of zircons from Petroleum Development Oman(PDO),Lahan-1, well gives a maximum age constraint of645Ma for the youngest glacial deposits.Furthermore, global correlation with glacial events suggest a cor-relative date of the Hadash cap carbonate of about 635Ma(Bowring et al.,2007).The Ara Group has several ash beds ranging in age from546±0.21Ma in the A0interval,to541.00±0.13Ma in the A4C(Bowring et al.,2007).Sedimentation rates from well-dated sections of the upper Buah Formation and Ara Group were extrapolated to give an approximate age of the basal Shuram Formation of554to562Ma.This analysis assumed a signifi-cant depositional hiatus at the Khufai-Shuram boun-dary and,thus,does not constrain the age of the Khufai Formation.The tectonic setting of the Nafun Group is also not well constrained.A suture zone in Yemen and arc-related volcanic basement of the Huqf suggests accretion of the Oman terrain onto the Arabian–Nubian plate prior to the deposition of Nafun strata. The Abu Mahara sediments and volcanics accumu-lated in extensional basins,whereas subsequent sedi-mentary deposition was regionally extensive and suggestive of relative tectonic quiescence(Allen, 2007).Accommodation for the Nafun Group was cre-ated primarily through thermal contraction of the crust in a passive margin sequence(Allen,2007). Others have argued that this mechanism is insuffi-cient to accommodate the whole of the Nafun Group and suggest dynamic lithospheric depression related to subduction(Grotzinger and Amthor,2002; Grotzinger et al.,2002).Significant transtensional tectonic and volcanic activity resumed during Ara deposition,creating the South Oman,Ghaba,and Fahud evaporite basins(Grotzinger and Amthor, 2002;Allen,2007;Bowring et al.,2007).The Nafun GroupPrevious work has focused on the deposition and sedimentary features of the Nafun Group as a whole (Gorin et al.,1982;McCarron,1999;Forbes et al., 2010),with additional studies focusing on aspects of the Masirah Bay(Allen and Leather,2006),Shuram (Le Guerroue et al.,2006a,b),and Buah(Cozzi and Grotzinger,2004)Formations,and Ara Group (Mattes and Conway Morris,1990).The Khufai Formation was initially interpreted as a uniformly shallow platform carbonate with abundant evaporitic collapse features(Gorin et al.,1982).Subsequent interpretations have favored a prograding ramp model with the basal strata representing subwave base depo-sition(Wright et al.,1990;McCarron,1999).The lower contact between the Masirah Bay Formation and the Khufai Formation is gradational,displaying interfingering of siltstones and carbonates over sev-eral meters of the contact(Allen and Leather,2006).Little agreement exists on the nature of the lithos-tratigraphic contact between the Khufai and Shuram Formations.Citing unpublished seismic data from Petroleum Development Oman and a potential local-ized karstic surface,some favor a significant deposi-tional hiatus and major sequence boundary (McCarron,1999;Bowring et al.,2007;Forbes et al., 2010).Others have noted minor hardground features in an oolite marking the uppermost Khufai Formation,but do not assign significant time to the boundary(Wright et al.,1990).The Khufai Formation represents a key interval in Earth history,capturing the onset of a global nega-tive excursion in the isotopic composition of carbon-ate carbon(Grotzinger et al.,2011),but has lacked complete characterization regarding its facies,stratig-raphy,and ramp evolution.This excursion was first observed in Oman(Burns and Matter,1993)and is initially recorded by the upper Khufai Formation, reaching a minimum in the overlying Shuram Formation,and increasing to the baseline well into the Buah Formation(Fike et al.,2006;Le Guerroue et al.,2006c).Multiple explanations for this excur-sion have been proposed,including some that have significant implications for the oxidation state of the global ocean and the evolution of the Ediacaran fauna (Grotzinger et al.,2011).Distribution of Khufai FormationThe Khufai Formation is best exposed in the Huqf area and in Jebel Akhdar.Continuous sections of the Khufai Formation exist in the Huqf area outcrop within three large anticlinal domes:the Mukhaibah, Khufai,and Buah domes(Figure3A).TheseO SBURN ET AL.1635locations and stratigraphic sections.(A)Geological map of the Huqf area show-ing the distribution of the Masirah Bay,Khufai,and Shuram Formations and domesare discussed in the text.(B)Sketch map ofthe Oman Mountains defined by the outerboundary of sedimentary uplift(gray).Erosional windows into the Precambrian areshown in white,and the Khufai Formationoutcrop is in dark s and paths ofmajor wadis are shown for reference.1636Stratigraphic Characterization of the Khufai Formation,Omanlocations feature generally good textural preservation and minimal structural deformation beyond the sim-ple,open folds that define the domes.Primary car-bonate sediments were pervasively dolomitized,which obscures fine depositional detail in strongly affected intervals.This contrasts with exposures in the core of the Jebel Akhdar uplift in the Oman Mountains (Figure 3B),where the Khufai Formation shows extensive ductile folding with associated pen-etrative cleavage,severe neomorphic recrystalliza-tion,and extensive faulting.The Khufai Formation in the Huqf area is approx-imately 300to 320m (1000to 1050ft)thick and forms a modern topographic high (Figure 4A).Both upper and lower contacts appear to be gradational,although the upper contact is rarely preserved.A regionally extensive,Tertiary-aged erosional event truncated the Khufai Formation preserved in the anti-clines and left erosional peneplanation surfaces mantled in thick siliceous breccias.Differential ero-sion of the Khufai Formation compared to the Masirah Bay and Shuram Formations commonly obscures the contacts and creates a steep gradient between the peneplanation surface and the valley floor 100m (320ft)below (Figure 4A).The Khufai Formation in the Oman Mountains is primarily composed of black,sulfurous limestone with gradational upper and lower contacts (Figure 4B).The lower and upper contacts are pre-served within mixed siliciclastic-carbonate transi-tional members that are consistently approximately 30m (100ft)thick.In contrast,the main phase of carbonate deposition shows significant thickness variability,ranging from 20to 122m (66to 400ft)(Figure 5).Although extensive folding adds error to thickness determinations,the stratigraphic variabil-ity appears to mimic primary bed thickness and grain-sizevariations.Figure 4.Interpreted outcrop photos from (A)Mukhaibah dome and (B)Al Aqor village in the Oman Mountains illustrating major facies groups.O SBURN ET AL .1637METHODSA total of eight stratigraphic sections covering the full Khufai Formation stratigraphy and seven shorter sec-tions were logged in the Huqf area,covering a modern plan view distance of approximately 85km (53mi)(Figure 3A).Five complete sections and one partial section were logged in the Oman Mountains,covering 55km (34mi)(Figure 3B).Stratigraphic sections document facies patterns,diagenetic over-prints,and parasequence stacking with approximately 10-cm (4-in.)resolution.Samples were collected for slab-scale textural analysis and petrographic analysis.The stratigraphic architecture of the Khufai Formation was analyzed in hierarchical order begin-ning with parasequences,followed by parasequence sets,and ramp architecture as a whole.Meter-scale parasequences are easily recognizable in outcropand combine to form a maximum of four orders of cycles,including parasequence,parasequence set (PS1),compound parasequence set (PS2),and sequence.Parasequences are the smallest observable scale of variability described from observations of accommodation minima and maxima within sets of depositional facies.The sequence is the largest scale of viability observed and corresponds to the formation scale.Ramp geometry was determined from facies and facies stacking patterns within the parasequences.FACIES AND FACIES TRACTS:HUQF AREA Depositional facies of the Khufai Formation in the Huqf area and the Oman Mountains differ signifi-cantly and are discussed separately.The Huqf area defines a broadly shallowing-upward rampsequence.Figure 5.Generalized stratigraphic columns from the Huqf and Oman Mountains illustrating low-resolution facies vari-ability and the position of major stratigraphic surfa-ces:MFI (dashed line),SB (solid line),TS (dot and dashed line).1638Stratigraphic Characterization of the Khufai Formation,OmanThe lower Khufai Formation is characterized by outer-ramp facies with little evidence for storm-wave reworking,whereas the upper Khufai Formation is characterized by peritidal and supratidal deposits. Following the nomenclature of Burchette and Wright(1992),we define outer-,middle-,and inner-ramp facies and group them into facies tracts follow-ing the model of Tinker(1998).In this usage,facies tracts contain genetically linked facies and facies suc-cessions that record a discrete energy,water depth, and sediment supply setting(Tinker,1998).The most distal facies form the outer-and middle-ramp facies tracts,whereas the inner ramp is more complex and is subdivided by energy and water depth into subtidal, variable-energy subtidal to intertidal,high-energy subtidal to intertidal,and intertidal to supratidal facies tracts.Finally,where the Khufai Formation includes abundant siliciclastic material,it is appropriate to erect a final facies tract:siliciclastic-influenced facies. The composition of these facies tracts and descrip-tions of individual facies are summarized in Table1 and detailed in the text below.Outer-Ramp Facies TractThe outer-ramp facies tract consists of peloidal intra-clast wackestone and packstone,thick-laminated mudstone,rare intraclast breccia,and siliciclastic silt-stone facies.These facies intercalate with those of the middle-ramp facies tract to form a90-to130-m(295-to437-ft)-thick,laterally continuous blanket that records the transgressive systems tract(TST)and maximum flooding interval(MFI)of the lower Khufai Formation.A continuous mineralogic grada-tion from underlying siliciclastic siltstones of the Masirah Bay is recorded by the transitional contact and intercalation of siltstone into overlying carbonate facies for the first∼10m(∼33ft)of the formation(Figure6A).The carbonate sediments reflect alternations between fallout of suspended fine carbonate mud and storm-remobilized density cur-rents containing inner-ramp–derived intraclasts. Peloidal Intraclast Wackestone and PackstonePeloidal intraclast wackestone or packstone comprises the bulk of the outer-ramp facies tract. Beds are meter scale and wavy,with scoured bases and cream-colored mudstone intraclasts millimeters to centimeters long concentrated at bed bases. The matrix is dark-gray dolomite mudstone or gray, peloidal,very fine grained packstone and was perva-sively dolomitized to a very fine interlocking crystal-line fabric(Figure6C).At the Buah dome,the intraclasts can be much larger(centimeter scale)and more abundant than in the Khufai and Mukhaibah domes grading into a matrix-supported pebble con-glomerate.This facies records downramp export of sediment by storm-generated currents consistent with a distal setting(Tucker and Wright,1990;Betzler et al.,1999).In this subwave base setting,density currents produced by storms would be sufficient to transport peloids and intraclasts from the inner-ramp to the outer-ramp environment(Schlager et al., 1994).Remobilization of pelagic mud accumulations creates the observed matrix components.Intraclasts show facies composition and early lithification more characteristic of inner-ramp shallower water facies and were thus likely eroded and incorporated into deeper water deposits(e.g.,Fairchild and Herrington, 1989).Thick-Laminated MudstoneThe thick-laminated mudstone forms meter-scale beds in the lower Khufai Formation outwardly resembling the wackestones described above. Laminae are centimeter scale and discontinuous, forming an irregular fabric that is highlighted by silicification and dedolomitization.No evidence for wave reworking exists.We interpret this facies to record a low-energy depositional environment below storm-wave base(McCarron,1999).Thin silty to micritic laminae separating thin packages were pre-viously interpreted as benthic microbial colonization (McCarron,1999);however,it is alternatively sug-gested here that this fabric represents pelagic fallout and accumulation of suspended organic material or detritus.Intraclast BrecciaIntraclast breccias are rare and consist of very coarse,matrix-supported breccia with laminated mudstone clasts(1to20cm[0.4to8in.]in length), occurring in1–5-m(3.3–16.5-ft)-thick laterally dis-continuous lenses.The matrix is muddy;however,O SBURN ET AL.1639T a b l e 1.F a c i e s a n d F a c i e s T r a c t s o f t h e K h u f a i F o r m a t i o n :H u q f A r e aF a c i e s L o c a t i o n *D e s c r i p t i o nI n t e r p r e t a t i o nO U T E R R A M P P e l o i d a l i n t r a c l a s t w a c k e s t o n e /p a c k s t o n eM ,K ,BD e c i m e t e r -t o m e t e r -s c a l e ,w a v y -b e d d e d r e c r y s t a l l i z e d d o l o m i t e m u d o r v e r y f i n e p e l o i d s w i t h m i n o r a n g u l a r m i c r i t i c o r s i l t y i n t r a c l a s t s .S c o u r e d b a s e s a r e c o m m o n a n d m u d s t o n e p a r t i n g s s e p a r a t e b e d s .D i s t a l c a r b o n a t e d e p o s i t i o n v i a d o w n r a m p t r a n s p o r t o f s t o r m r e m o b i l i z e d s e d i m e n t a n d i n t r a c l a s t s .T h i c k -l a m i n a t e d m u d s t o n e M ,K ,BT h i c k -b e d d e d p a c k a g e s o f m i l l i m e t e r -t o c e n t i m e t e r -s c a l e c a r b o n a t e m u d s t o n e i r r e g u l a r l a m i n a e w i t h a f e w s l u m p s a n d /o r s c o u r s .O f t e n s t r o n g l y p e t r o l i f e r o u s .D i s t a l d e p o s i t i o n o f s u s p e n d e d c a r b o n a t e f i n e s .I n t r a c l a s t b r e c c i aK (r a r e ),M (r a r e )M a t r i x -s u p p o r t e d b r e c c i a s w i t h c e n t i m e t e r -t o d e c i m e t e r -s c a l e ,i n n e r -r a m p -d e r i v e d l a m i n a t e d m u d s t o n e c l a s t s .L o c a l d e b r i s f l o w s r e s u l t i n g f r o m c o l l a p s e o f l o c a l r e l i e f o n t h e r a m p .S i l i c i c l a s t i c s i l t s t o n eM ,K ,BY e l l o w ,p i n k ,o r r e d c a r b o n a t e -c e m e n t e d s i l t s t o n e i n t e r b e d d e d w i t h d e c i m e t e r -s c a l e c a r b o n a t e b e d s .L a m i n a e a l t e r n a t e b e t w e e n m i x e d m i n e r a l o g y s i l t s t o n e a n d c l a y i n t h i n s e c t i o n .P e l a g i c f a l l o u t o f s u s p e n d e d s i l i c i c l a s t i c f i n e s a n d c l a y .M I D D L E R A M P H u m m o c k y c r o s s -s t r a t i f i e d p a c k s t o n eM ,K ,BP e l o i d p a c k s t o n e w i t h c o m m o n h u m m o c k y c r o s s -s t r a t i f i c a t i o n a n d s o f t s e d i m e n t d e f o r m a t i o n .S m a l l e r b e d s a r e o f t e n a m a l g a m a t e d i n t o m e t e r -s c a l e p a c k a g e s .T r a n s p o r t a n d d e p o s i t i o n o f f i n e -g r a i n e d s e d i m e n t s d u r i n g s t o r m s .S i l t y p l a n a r -l a m i n a t e d m i c r i t e M ,KM i l l i m e t e r -t o c e n t i m e t e r -s c a l e ,u n i f o r m l y p l a n a r -b e d d e d m u d s t o n e .S c o u r s ,s l u m p s ,c o n v o l u t e b e d d i n g ,a n d r i p -u p l e n s e s .F a l l o u t o f s u s p e n d e d f i n e s ;s u b s e q u e n t r e w o r k i n g b y s t o r m w a v e s .B l a c k a n d t a n l a m i n i t eBA s a b o v e w i t h d a r k o r g a n i c m a t t e r a n d p y r i t e -r i c h l a m i n a e a l t e r n a t i n g w i t h w h i t e s i l t y r e c r y s t a l l i z e d l i m e s t o n e .P e t r o l i f e r o u s .F a l l o u t o f s u s p e n d e d f i n e s i n t o a n o x i c e n v i r o n m e n t .S t o r m w a v e r e w o r k i n g w i t h v a r i a b l e s i l i c i c l a s t i c s e d i m e n t s u p p l y .I N N E R R A M P S u b t i d a l P e l o i d a l t o c o a t e d -g r a i n g r a i n s t o n eM ,K ,BW e l l s o r t e d ,v e r y f i n e t o f i n e p e l o i d a l t o c o a t e d -g r a i n g r a i n s t o n e i n d e c i m e t e r -s c a l e b e d s w i t h a b u n d a n t t a b u l a r a n d f e s t o o n c r o s s -s t r a t i f i c a t i o n (10c m [4i n .]w a v e l e n g t h ).M u d -d r a p e d r i p p l e s a n d a f e w o n c o l i t e g r a i n s .G r a i n s t o n e b a r r i e r s h o a l c o m p l e x w i t h c o n s t a n t t i d a l r e w o r k i n g .I n t r a c l a s t d o l o s i l t i t e MB l o c k y 2–5-c m (0.8–2-i n .)-t h i c k b e d s o f f i n e c r e a m -c o l o r e d d o l o m i t e w i t h l a r g e -s c a l e h u m m o c k y c r o s s -s t r a t i f i c a t i o n a n d s e l f -s o u r c e d p e b b l e -s i z e i n t r a c l a s t s .R e s e d i m e n t e d o o i d s i n f i r s t b e d ,i n c r e a s i n g r e d s i l t u p w a r d .F l o o d i n g o f t h e p l a t f o r m t o a h i g h -e n e r g y s u b t i d a l e n v i r o n m e n t .1640Stratigraphic Characterization of the Khufai Formation,OmanL o w -E n e r g y S u b t i d a l t o I n t e r t i d a l O n c o l i t e g r a i n s t o n e /b o u n d s t o n e M ,K M e d i u m t o v e r y c o a r s e o n c o l i t e g r a i n s (e l l i p t i c a l a n d 1–5m m i n d i a m e t e r )i n c h a n n e l s a n d 10–50-c m (4–20-i n .)-t h i c k b e d s w i t h c r o s s -b e d d i n g ,r i p p l e s ,a n d b a s a l s c o u r s .G r a i n s h a v e i r r e g u l a r l y c o n c e n t r i c m i c r i t i c r i m s .F e w m i c r o b i a l l a m i n a t i o n s .H e a v i l y s i l i c i f i e d .C o a t e d g r a i n s f o r m i n g i n s i t u i n a n i n n e r -r a m p ,w a v e -a n d t i d e -a g i t a t e d e n v i r o n m e n t .P e r i o d s o f q u i e s c e n c e a l l o w f o r m i c r o b i a l b i n d i n g .P e r i t i d a l M i c r o b i a l i t e I r r e g u l a r l a m i n i t e t o s t r o m a t o l i t eM ,KI r r e g u l a r l a m i n a t i o n s i n p e l o i d a l t o m i c r i t i c m a t r i x ,c o m m o n b l i s t e r s a n d l o c a l r e l i e f i n t o c e n t i m e t e r -s c a l e i r r e g u l a r s t r o m a t o l i t e s .T h i c k -b e d d e d (d m )o r a s l a y e r s w i t h i n g r a i n s t o n e f a c i e s .I n t e r b e d d e d o n c o l i t e g r a i n s t o n e l e n s e s ;g e n e r a l l y s i l i c i f i e d .L i t h i f i e d s t r a t i f o r m t o d o m a l m i c r o b i a l m a t s .L o c a l g r o w t h o f s m a l l s t r o m a t o l i t e s w h e r e t h e r e i s s u f f i c i e n t s u s t a i n e d w a t e r d e p t h .P e r i o d i c e x p o s u r e .T u f t e d l a m i n i t e t o s t r o m a t o l i t e M ,KI r r e g u l a r l a m i n a t i o n s f o r m i n g t u f t s w i t h ∼1c m s y n o p t i c r e l i e f ,s p a c e d 3–10c m .C o a t e d g r a i n s f i l l d a m p r e l i e f .O r g a n i z e d m i c r o b i a l m a t w i t h p e r i o d i c w a s h o v e r o f c o a r s e r g r a i n s .F l a s e r -b e d d e d p a c k s t o n eBF i s s i l e c e n t i m e t e r -s c a l e b e d d e d p a c k a g e s o f r i p p l e d c a r b o n a t e s a n d (p e l o i d s ,o o i d s ,c o a t e d g r a i n s ,o o l i t e f r a g m e n t s ,a n d m u d s t o n e c h i p s )a n d i n t e r b e d d e d m u d s t o n e s ;f e t i d .I n t e r t i d a l t o s u b t i d a l e n v i r o n m e n t w i t h a l t e r n a t i n g r e l a t i v e h i g h a n d l o w w a v e e n e r g y .L a g o o n a l F i n e s M u d s t o n e M ,K C e n t i m e t e r -t o d e c i m e t e r -b e d d e d ,c r e a m -c o l o r e d c a r b o n a t e m u d ,l o c a l t h i n p l a n a r l a m i n a t i o n s .R e s t r i c t e d s h a l l o w p o o l s t o l a g o o n s w i t h i n t i d a l -f l a t m o s a i c .L a m i n a t e d d o l o s i l t i t e M ,KS i l t y p l a n a r -l a m i n a t e d w h i t e c a r b o n a t e .L o c a l l o w s t r o m a t o l i t e s ,g y p s u m l a t h e s ,a n d s i l t l a m i n a e .L o w -e n e r g y r e s t r i c t e d l a g o o n .H i g h -E n e r g y S u b t i d a l t o I n t e r t i d a l M i x e d g r a i n g r a i n s t o n e /p a c k s t o n eM ,K F i n e p e l o i d s ,s m a l l o n c o l i t h s ,o o i d s ,a n d /o r c o a t e d g r a i n s i n c e n t i m e t e r -s c a l e r i p p l e d o r p l a n a r -l a m i n a t e d p a c k a g e s ,c o m m o n l y s e p a r a t e d b y t h i n m i c r o b i a l l a m i n i t e d r a p e s a n d s i l i c i f i e d i n b a n d s .B e d d i n g g e n e r a l l y 5t o 10c m (2t o 4i n .)a l t h o u g h o u t s i z e d b e d s (∼50c m [∼20i n .])o c c u r .H i g h -e n e r g y i n t e r t i d a l g r a i n s t o n e .L o c a l t i d a l b a r s o r s a n d w a v e s .S m o o t h -l a m i n a t e d s t r o m a t o l i t e M ,KM e t e r -s c a l e l o w s y n o p t i c r e l i e f d o m a l t o c o n i c a l s t r o m a t o l i t e s .E l o n g a t e i n p l a n v i e w .L a m i n a e a r e s m o o t h e v e n c o a t s .M i c r o b i a l m a t s f o r m i n g s t r o m a t o l i t e s i n t h e l o w e r i n t e r t i d a l z o n e .O o i d i n t r a c l a s t g r a i n s t o n eM ,K ,BS p h e r i c a l t o s t r o n g l y e l o n g a t e o o i d s a n d c o m p o s i t e g r a i n s w i t h r a d i a l f i b r o u s e a r l y c e m e n t s .I n t r a c l a s t s a r e c o m p o s e d o f m u d s t o n e ,m u d d y s t r o m a t o l i t e p i e c e s ,a n d r e s e d i m e n t e d o o l i t e .H i g h -e n e r g y s h o a l c o m p l e x f o r m e d d u r i n g l a r g e -s c a l e t r a n s g r e s s i o n i n t o t h e S h u r a m F o r m a t i o n .M u d d y s t r o m a t o l i t e M ,KU p t o 2-m (6.6-f t )s y n o p t i c r e l i e f ,0.5t o 5m (1.6–16f t )d i a m e t e r ,b i o h e r m s f o r m e d f r o m i r r e g u l a r s t r o m a t o l i t e s w i t h c o l u m n a r ,d o m a l ,a n d c o n i c a l m o r p h o l o g i e s a l l w i t h t h i c k 2m m (0.08i n .)t o 1c m (0.4i n .)m i c r i t i c l a m i n a e .M i c r o b i a l m o u n d s n u c l e a t i n g w i t h i n t h e o o l i t e d u r i n g f l o o d i n g i n t o t h e S h u r a m F o r m a t i o n .(c o n t i n u e d )O SBURN ET AL .1641。