Episodic sedimentation on an early Silurian, storm-dominated carbonate ramp, Becscie and Merrimack formations, Anticosti Island, Canada

The 150–160 m thick lowermost Silurian (Rhuddanian) Becscie and Merrimack formations of Anticosti Island, Canada, represent continuous deposition on a shallow, open marine carbonate ramp. Several rock types are identified: (a) laminated and homogenous mudstone; (b) laminated and homogenous packstone; (c) argillaceous mudstone and packstone; (d) calcareous shale; (e) laminated calcisiltite; (f) medium- to fine-grained grainstone; and (g) bio/intraclastic rudstone. These rock types are arranged into five distinct lithofacies: (LF1) calcareous mudstone-shale; (LF2) laminated-homogenous mudstone; (LF3) calcareous grainstone-shale; (LF4) laminated mudstone-grainstone; and (LF5) laminated calcisiltite-grainstone. The sequence reflects deposition on a low-energy, muddy, carbonate to argillaceous ramp subject to short-lived, episodic high-energy storms. These events produced fining-upwards storm units 5–80 cm thick, or tempestites, consisting of: a sharp scoured base overlain by intra/bioclastic rudstone grading upwards into medium-grained grainstone, finely laminated calcisiltite and mudstone, or shale. These are interbedded with low-energy, fairweather mudstones and calcareous shales. Deposition progressed from a carbonate mud-dominated ramp in the Becscie Formation to an argillaceous mud-dominated ramp in the Merrimack Formation. Lateral tempestite proximality trends and lithofacies distribution indicate that the Anticosti Basin deepened to the south-east into the Iapetus Ocean and shallowed towards a SW—NE-orientated shoreline to the north-west. Vertical tempestite proximality trends and lithofacies changes identify third-order eustatic sea-level changes. After an initial deepening at the base of the formation, a shallowing-deepening event dominated the sequence. Several higher order fluctuations, defined by lithofacies and tempestite proximality trends, are superimposed on these changes. The fluctuations identified with the aid of tempestite proximality trends are of an order of magnitude higher than those identified by either lithofacies or palaeontological methods.     

西藏改则热那错上三叠统日干配错组碳酸盐岩岩石学特征与沉积环境*

西藏改则县热那错东沟剖面上三叠统卡尼阶至瑞替阶日干配错组沉积了厚度较大的碳酸盐岩地层,其中化石丰富。在碳酸盐岩中识别出11种主要的岩石类型：灰泥灰岩﹑含生物碎屑灰泥灰岩、生物碎屑粒泥灰岩、生物碎屑泥粒灰岩、内碎屑泥粒灰岩、内碎屑颗粒灰岩、藻颗粒灰岩、多种类型鲕粒灰岩、单一类型鲕粒灰岩、球粒泥粒灰岩和生物礁灰岩。根据岩石特征及组合类型可划分为5种沉积相：陆源碎屑滨岸相、局限台地相、开阔台地相、台地边缘浅滩相和台地边缘礁相,它们共同构成了日干配错组4个有序的海侵—海退旋回,整体显现出海侵的相序结构。     

重庆万盛中二叠统碳酸盐岩微相研究

重庆市万盛区中二叠统碳酸盐岩分布广泛，露头发育良好，发育有10种微相类型，即灰泥石灰岩、筵粒泥灰岩、球粒生物碎屑粒泥灰岩、生物碎屑内碎屑泥粒灰岩、荷叶藻泥粒灰岩、鲕粒内碎屑泥粒岩/颗粒岩、小有孔虫泥粒灰岩、生物碎屑泥粒灰岩、绿藻颗粒灰岩和生物碎屑颗粒灰岩。综合分析表明，研究区中二叠统形成于滨岸、局限台地和开阔台地环境，据此建立了沉积相模式。     

Distribution of Chancelloriids in a Middle Cambrian Carbonate Platform Deposit, Taebaek Group, Korea

The onset of the Cambrian witnessed the diversification of “small shelly fossils (SSF)”, which affected carbonate depositional system. One of the problematic SSF, chancelloriids, are common components of the early to middle Cambrian carbonate and shale, and their contributions toward Cambrian carbonates are not yet fully understood. This study assesses distribution patterns of chancelloriid sclerites in the Cambrian Series 3 Daegi Formation based on microfacies analysis and discusses their sedimentologic implications. In the lower part of the formation, partially articulated chancelloriid sclerites occur mainly in bioclastic packstone and grainstone facies, with isolated sclerite rays in nodular packstone to grainstone facies. In the middle part of the formation, chancelloriid fragments occur only sporadically in bioclastic wackestone to packstone, bioclastic grainstone and oolitic packstone to grainstone facies, whereas boundstone facies are nearly devoid of their fragments. There are no chancelloriid fragments in the upper part of the formation, which consists of oolitic packstone to grainstone facies. Chancelloriids are interpreted to have primarily occupied platform margin shoal environments, shedding their sclerites to surrounding areas, and thus contributed as sediment producers. The distribution of Daegi chancelloriid sclerites is similar to other Cambrian examples, with the exception of common chancelloriids in Cambrian Series 2 reefs and their apparent near absence in the Daegi and other Cambrian Series 3 reefs. This disparity resulted from changes in the ecologic niche of chancelloriids after the end-Cambrian Series 2 reef crisis, coupled with an overall decline of chancelloriids in the middle Cambrian.     

云南永德鱼塘寨石炭－二叠系界线剖面的生物地层学及沉积环境

云南永德鱼塘寨石炭－二叠界线剖面沉积连续、化石丰富，可作为滇西地区的代表性剖面，自石洞寺组至草坝山头组底部灰岩可划分出１６种主要的碳酸盐微相类型，分别形成于外陆架、中陆架和内陆架环境。     

贵州紫云晚石炭世叶状藻礁灰岩微相特征与沉积环境的研究

紫云晚石炭世叶状藻礁发育在碳酸盐岩台地边缘内。研究以碳酸盐岩微相分析为主要手段,对叶状藻礁剖面中岩石的微相特征和沉积环境研究。主要总结出6个微相类型,为生物碎屑粒泥灰岩、生物碎屑泥粒灰岩、球粒生屑泥粒灰岩、叶状藻粒泥灰岩—叶状藻泥粒灰岩、腕足泥粒灰岩、生物碎屑颗粒灰岩。总结了一个在开阔台地与台地边缘过渡带上的局部相模式,由较深水的开阔台地相、叶状藻礁相、台内斜坡相和台内较浅水的斜坡相、较浅水的开阔台地相、台内浅滩相过渡,分析总结了叶状藻礁的沉积环境。微相组合及其在剖面纵向分布规律显示,剖面的总体沉积环境为潮下带浅水开阔台地环境。      

Paleoenvironment of deposition of the Dupi Tila Formation,Lalmai Hills,Comilla, Bangladesh

The Dupi Tila Formation is composed of yellow to light brown medium to very fine moderately hard to loose sandstone, siltstone, silty clay, mudstone and shale with some conglomerates with clasts of petrified wood. The lithofacies of matrix supported conglomerate, trough cross bedded conglomerate, massive sandstone, trough cross bedded sandstone, planar cross bedded sandstone, ripple cross laminated sandstone-siltstone, flaser laminated sandstone-shale, lenticular laminated sandstone-siltstone-shale, parallel laminated sandstone-siltstone, wavy laminated shale, parallel laminated blue shale, and mudstone are delineated within this formation. Based on the grain size, sedimentary structures, water depth and genesis of individual facies, facies are grouped into three types of facies associations like (i) coarse-grained conglomerate facies association in relation to tractive current deposits of alluvial fan set up at the base of litho-succession (FAC), (ii) medium to fine-grained sandstone-siltstone-mudstone facies association or facies association in relation to strong tide (FAT) characterizing the middle part of litho-succession, (iii) very fine-grained sandstone-siltstone-mudstone facies association in relation to less frequent weak tide or heterolithic facies association (FAHL) characterizing upper part of litho-succession and shallow marine facies association (FASM) composing the uppermost litho-succession. Presence of gluconite indicates that the depositional environment was shallow to deep marine. The dominant paleoflow direction during the deposition of Dupi Tila Formation was toward southeast to southwestern direction. The rivers were of braided type at the piedmont alluvial depositional set up at the lower part, which later changed to estuarine-tidal flat type environmental set up in the middle part to upper part and paleo-environment was shallow marine in the uppermost part.     

陕西镇安西口石炭系/二叠系界线剖面碳酸盐岩微相特征与沉积环境的研究

在陕西镇安西口从上石炭统逍遥阶和下杨家河阶到下二叠统上杨家河阶、范家河阶、垭口阶和隆林阶的地层剖面上,区分出10种碳酸盐岩微相类型,即含生物碎屑灰泥灰岩、生物碎屑粒泥灰岩、生物碎屑泥粒灰岩、粒泥灰岩—泥粒灰岩、粗枝藻粒泥灰岩—粗枝藻泥粒灰岩、腕足棘皮粒泥灰岩—腕足棘皮泥粒灰岩、生物碎屑颗粒灰岩、团块颗粒灰岩、核形石颗粒灰岩和含鲕粒团块颗粒灰岩。它们形成于开阔台地和台地边缘浅滩2种沉积环境,开阔台地又进一步分为较浅水的开阔台地浅滩和较深水的开阔台地滩间,台地边缘浅滩内则局部发育藻丘。在此基础上,识别出在晚石炭世逍遥期至早二叠世隆林期研究区发生过12次相对海平面波动,它们构成5次明显的相对海平面升降;最后讨论了三里冲剖面上各阶界线与海平面变化的关系。     

An overview of the diagenesis of the Upper Jurassic carbonates of Jubaila and Hanifa formations, central Saudi Arabia

The Jubaila Formation (Upper Jurassic) in central Saudi Arabia has been divided into lower, middle, and upper parts purely on lithologic grounds. Each part consists of a major lower unit of lime mudstone and a minor upper unit of grainstone. This persistent change in the limestone facies is interpreted as a reflection of repeated shoaling up in the depositional shelf environment. It is a normal marine carbonate sequence that varies in thickness from 85 to 126 m. In the Hanifa Formation, the lowermost brown ledges in the section comprise a series of coarsening upward sequences which generally terminate in a fossiliferous/peloidal packstone and grainstone and subordinately lime mudstone facies. The middle slope member is yellow, blocky weathered shale and marl. Above this slope member are several thick beds of brown-coated fossiliferous wackestone, packstone, and grainstone with the association of lime mudstone in certain levels. These are fairly resistant ledges due to the occurrence of stromatoporoids. Dedolomitization occurs in the Jubaila Formation in various textural forms which include composite calcite rhombohedra, zonal dedolomitization, regeneration of predolomitization fabric of the limestone, and coarsely crystalline calcite mosaics with or without ferric oxide rhombic zones. Rhombohedral pores commonly occur in intimate association with dolomite, possibly resulting from the leaching of calcitized dolomite rhombohedra. The regional dedolomitization was most likely brought about by calcium sulfate solutions reacting with dolomites. The source of sulfate solutions is the dissolved anhydrite deposits of the Arab–Hith Formations, sometime before their erosion, and it takes place at or near an exposed surface. The Hanifa Formation shows various diagenetic features. These include dolomitization, dedolomitization, micritization, cementation, and recrystallization. Most of the examined samples of the Hanifa carbonates are dolomitized and subsequently dedolomitized as evidenced by the presence of iron-coated dolomite rhombs partially or completely calcitized. Dolomite also occurs in the lime mudstone, wackestone, packstone, and grainstone facies, while leaching of wackestone and packstone and dedolomitization of dolomite and dolomitic limestone followed by recrystallization are common processes.     

The Cretaceous Uhangri Formation, SW Korea: lacustrine margin facies

The Uhangri Formation forms part of the Cretaceous sedimentary sequence deposited in a series of inland basins in the south-western Korean Peninsula. It comprises an approximately 400-m-thick epiclastic sequence of conglomerate, (gravelly) sandstone, cherty mudstone and black shale. The entire sequence can be represented by 16 distinctive sedimentary facies organized into four facies associations. Facies association I is characterized by thick homogeneous brownish siltstone, wedge-shaped disorganized conglomerate and thinly interlayered gravelly sandstone units. The siltstone units were formed by large floods submerging the alluvial fan fringe (floodplain), whereas the conglomerate and gravelly sandstone units were deposited by sheetfloods and debris flows. Facies association II consists of stratified conglomerate — gravelly sandstone, laminated sandstone and sandstone/siltstone couplets which form fining-upward cycles. Some facies units are low-angle trough cross-bedded and show broad channel geometries. This association represents subaqueous delta lobes fed by high- and low-concentration turbidity currents in the distal delta realm. Facies association III is characterized, by wedged conglomerate and gravelly sandstone facies with interfingered massive sandstone bounded by scoured bases. It represents a delta front where distributary channels and mouth bars are dominant. Facies association IV consists of laterally continuous sequence of laminated black shale, crudely stratified sandstone and convoluted sandstone/cherty mudstone. This facies association is suggestive of depositional processes controlled by chemical equilibrium resulting from an interaction between density inflows and lake water. The cherty mudstone resulted from inorganic precipitation from siliceous solution provided by acidic volcanism. The Uhangri sequence generally shows a fining-upward trend with a transition from alluvial fan fringe, coarse-grained subaqueous delta, to shallow lake. The retrogradation was probably due to continuous subsidence related to continental rifting in the oblique-slip mobile zone.     

Sedimentology,sequence stratigraphy and syn-rift model of younger part of Washtawa Formation and early part of Kanthkot Formation,Wagad, Kachchh basin,Gujarat

The 600 m thick prograding sedimentary succession of Wagad ranging in age from Callovian to Early Kimmeridgian has been divided into three formations namely, Washtawa, Kanthkot and Gamdau. Present study is confined to younger part of the Washtawa Formation and early part of the Kanthkot Formation exposed around Kanthkot, Washtawa, Chitrod and Rapar. The depositional architecture and sedimentation processes of these deposits have been studied applying sequence stratigraphic context. Facies studies have led to identification of five upward stacking facies associations (A, B, C, D, and E) which reflect that deposition was controlled by one single transgressive — regressive cycle. The transgressive deposit is characterized by fining and thinning upward succession of facies consisting of two facies associations: (1) Association A: medium — to coarse-grained calcareous sandstone — mudrocks alternations (2) Association B: fine-grained calcareous sandstone — mudrocks alternations. The top of this association marks maximum flooding surface as identified by bioturbational fabrics and abundance of deep marine fauna (ammonites). Association A is interpreted as high energy transgressive deposit deposited during relative sea level rise. Whereas, facies association B indicates its deposition in low energy marine environment deposited during stand-still period with low supply of sediments. Regressive sedimentary package has been divided into three facies associations consisting of: (1) Association C: gypsiferous mudstone-siltstone/fine sandstone (2) Association D: laminated, medium-grained sandstone — siltstone (3) Association E: well laminated (coarse and fine mode) sandstone interbedded with coarse grained sandstone with trough cross stratification. Regressive succession of facies association C, D and E is interpreted as wave dominated shoreface, foreshore to backshore and dune environment respectively. Sequence stratigraphic concepts have been applied to subdivide these deposits into two genetic sequences: (i) the lower carbonate dominated (25 m) transgressive deposits (TST) include facies association A and B and the upper thick (75m) regressive deposits (HST) include facies association C, D and E. The two sequences are separated by maximum flooding surface (MFS) identified by sudden shift in facies association from B to C. The transgressive facies association A and B represent the sediments deposited during the syn-rift climax followed by regressive sediments comprising association C, D and E deposited during late syn-rift stage.     

Depositional facies,environments and sequence stratigraphic interpretation of the Middle Triassic–Lower Cretaceous (pre-Late Albian) succession in Arif El-Naga anticline,northeast Sinai,Egypt

The Middle Triassic–Lower Cretaceous (pre-Late Albian) succession of Arif El-Naga anticline comprises various distinctive facies and environments that are connected with eustatic relative sea-level changes, local/regional tectonism, variable sediment influx and base-level changes. It displays six unconformity-bounded depositional sequences. The Triassic deposits are divided into a lower clastic facies (early Middle Triassic sequence) and an upper carbonate unit (late Middle- and latest Middle/early Late Triassic sequences). The early Middle Triassic sequence consists of sandstone with shale/mudstone interbeds that formed under variable regimes, ranging from braided fluvial, lower shoreface to beach foreshore. The marine part of this sequence marks retrogradational and progradational parasequences of transgressive- and highstand systems tract deposits respectively. Deposition has taken place under warm semi-arid climate and a steady supply of clastics. The late Middle- and latest Middle/early Late Triassic sequences are carbonate facies developed on an extensive shallow marine shelf under dry-warm climate. The late Middle Triassic sequence includes retrogradational shallow subtidal oyster rudstone and progradational lower intertidal lime-mudstone parasequences that define the transgressive- and highstand systems tracts respectively. It terminates with upper intertidal oncolitic packstone with bored upper surface. The next latest Middle/early Late Triassic sequence is marked by lime-mudstone, packstone/grainstone and algal stromatolitic bindstone with minor shale/mudstone. These lower intertidal/shallow subtidal deposits of a transgressive-systems tract are followed upward by progradational highstand lower intertidal lime-mudstone deposits. The overlying Jurassic deposits encompass two different sequences. The Lower Jurassic sequence is made up of intercalating lower intertidal lime-mudstone and wave-dominated beach foreshore sandstone which formed during a short period of rising sea-level with a relative increase in clastic supply. The Middle-Upper Jurassic sequence is represented by cycles of cross-bedded sandstone topped with thin mudstone that accumulated by northerly flowing braided-streams accompanying regional uplift of the Arabo–Nubian shield. It is succeeded by another regressive fluvial sequence of Early Cretaceous age due to a major eustatic sea-level fall. The Lower Cretaceous sequence is dominated by sandy braided-river deposits with minor overbank fines and basal debris flow conglomerate.     

贵阳乌当地区中、上奥陶统沉积学研究

贵阳乌当地区上奥陶统碳酸盐岩地层与扬子地区同时代地层相比具有其特殊性,黄花冲组与宝塔组"龟裂纹"灰岩时代相当.通过对该地区中、上奥陶统碳酸盐岩的研究,划分出亮晶生屑灰岩相、泥晶生屑灰岩相、含生屑泥晶灰岩相、生屑泥晶灰岩相和页岩-粉砂岩相5种沉积相;根据沉积相组合推断此时期乌当地区黄花冲组处于局限潮下低能环境.     

The Late Jurassic Tithonian, a greenhouse phase in the Middle Jurassic–Early Cretaceous 'cool' mode: evidence from the cyclic Adriatic Platform, Croatia

Well‐exposed Mesozoic sections of the Bahama‐like Adriatic Platform along the Dalmatian coast (southern Croatia) reveal the detailed stacking patterns of cyclic facies within the rapidly subsiding Late Jurassic (Tithonian) shallow platform‐interior (over 750 m thick, ca 5–6 Myr duration). Facies within parasequences include dasyclad‐oncoid mudstone‐wackestone‐floatstone and skeletal‐peloid wackestone‐packstone (shallow lagoon), intraclast‐peloid packstone and grainstone (shoal), radial‐ooid grainstone (hypersaline shallow subtidal/intertidal shoals and ponds), lime mudstone (restricted lagoon), fenestral carbonates and microbial laminites (tidal flat). Parasequences in the overall transgressive Lower Tithonian sections are 1–4·5 m thick, and dominated by subtidal facies, some of which are capped by very shallow‐water grainstone‐packstone or restricted lime mudstone; laminated tidal caps become common only towards the interior of the platform. Parasequences in the regressive Upper Tithonian are dominated by peritidal facies with distinctive basal oolite units and well‐developed laminate caps. Maximum water depths of facies within parasequences (estimated from stratigraphic distance of the facies to the base of the tidal flat units capping parasequences) were generally <4 m, and facies show strongly overlapping depth ranges suggesting facies mosaics. Parasequences were formed by precessional (20 kyr) orbital forcing and form parasequence sets of 100 and 400 kyr eccentricity bundles. Parasequences are arranged in third‐order sequences that lack significant bounding disconformities, and are evident on accommodation (Fischer) plots of cumulative departure from average cycle thickness plotted against cycle number or stratigraphic position. Modelling suggests that precessional sea‐level changes were small (several metres) as were eccentricity sea‐level changes (or precessional sea‐level changes modulated by eccentricity), supporting a global, hot greenhouse climate for the Late Jurassic (Tithonian) within the overall ‘cool’ mode of the Middle Jurassic to Early Cretaceous.     

Ichnology of Bhuban and Boka Bil Formations,oligocene-miocene deposits of Manipur Western Hill,Northeast India

Oligocene-Miocene deposits of Bhuban and Boka Bil Formations, Surma Group, Manipur Western Hill consist of well preserved ichnofossil assemblages. These formations are represented by eight lithofacies such as Massive sandstone (Sm), Rippled marked argillaceous sandstone (Sr), Wavy laminated sandstone-siltstone-silty shale (Sw), Laminated shale (Fl), Massive mudstone (Fm), Trough cross-bedded sandstone (St), Lenticular laminated sandstonesiltstonesilty shale (Sll) and Laminated to massive sandstone-siltstone (Ssc). Fifteen ichnospecies were identified, which further categories into Skolithos, Cruziana, and Skolithos/Cruziana ichnofacies. Overall distribution pattern and behavioural nature of the ichnoassemblage and sedimentological attributes suggests that the sediments of Bhuban and Boka Bil Formations were deposited under frequent fluctuating sea level, moderate to strong energy condition, subtidal to lower intertidal environment, rich in organic nutrients.     

重庆老龙洞二叠系-三叠系界线地层的海平面下降事件

老龙洞的二叠系-三叠系界线地层剖面许多学者做过研究,关于其海平面变化问题的争论一直没有解决。本文的研究首次在该剖面上发现了明显的侵蚀面,说明曾经有海平面下降事件。对沉积环境的解释也有新的认识。该剖面在长兴期钙质海绵灰岩之上沉积了一套开阔台地相的沉积,其上是1.4m 厚的局限台地相沉积,具有斑点状构造。此层顶部出现一个波状起伏的侵蚀面。侵蚀面上下的岩性不同。此面之上是0.8m 厚的浅水的局限台地相沉积,具有树枝状的构造和外貌。此层顶面是一个更加明显的侵蚀面,起伏高差达到0.3m 以上。侵蚀面上下的岩性截然不同,以及其上岩石层理与侵蚀面的斜交关系,以及侵蚀面的形态把侵蚀面同缝合线区分开来。这个侵蚀面代表一次显著海平面下降事件之后的一次较长时间的出露和风化剥蚀。侵蚀面之上是透镜状分布的灰色的层状的小腹足类的颗粒岩,再上是一薄层灰黄色的泥粒岩,含丰富的小双壳类。此层以上是青灰色的薄板状的泥质灰泥岩,舍丰富的0.3～3mm 大小的、内部重结晶的同生角砾。     

An iron-bearing wave-dominated siliciclastic shelf: Facies analysis and palaeogeographic implications (Silurian-Lower Devonian Sierra Grande Formation,Southern Argentina)

The Sierra Grande Formation (Silurian-Early Devonian) consists of quartz arenites associated with clast supported conglomerates, mudstones, shales and ironstones. Eight sedimentary facies are recognized: cross-stratified and massive sandstone, plane bedded sandstone, ripple laminated sandstone, interstratified sandstone and mudstone, laminated mudstone and shale, oolitic ironstone, massive conglomerate and sheet conglomerate lags. These facies are interpreted as shallow marine deposits, ranging from foreshore to inner platform environments. Facies associations, based on vertical relationships among lithofacies, suggest several depositional zones: (a) beach to upper shoreface, with abundant plane bedded and massive bioturbated sandstones; (b) upper shoreface to breaker zone, characterized by multistorey cross-stratified and massive sandstone bodies interpreted as subtidal longshore-flow induced sand bars; (c) subtidal, nearshore tidal sand bars, consisting of upward fining sandstone sequences; (d) lower shoreface zone, dominated by ripple laminated sandstone, associated with cross-stratified and horizontal laminated sandstone, formed by translatory and oscillatory flows; and (e) transitional nearshore-offshore and inner platform zones, with heterolithic and pelitic successions, and oolitic ironstone horizons. Tidal currents, fair weather waves and storm events interacted during the deposition of the Sierra Grande Formation. However, the relevant features of the siliciclastics suggest that fair weather and storm waves were the most important mechanisms in sediment accumulation. The Silurian-Lower Devonian platform was part of a continental interior sag located between southern South America and southern Africa. The Sierra Grande Formation was deposited during a second order sea level rise, in which a shallow epeiric sea flooded a deeply weathered low relief continent.     

Sedimentology of a lacustrine fan-delta system, Miocene Horse Camp Formation, Nevada, USA

A 1600-m-thick succession of the Miocene Horse Camp Formation (Member 2) exposed in east-central Nevada records predominantly terrigenous clastic deposition in subaerial and subaqueous fan-delta environments and nearshore and offshore lacustrine environments. These four depositional environments are distinguished by particular associations of individual facies (14 defined facies). Subaerial and subaqueous fan-delta facies associations include: ungraded, matrix-and clast-supported conglomerate; normally graded, matrix- and clast-supported conglomerate; ungraded and normally graded sandstone; and massive to poorly laminated mudstone. Subaqueous fan-delta deposits typically have dewatering structures, distorted bedding and interbedded mudstone. The subaerial fan-delta environment was characterized by debris flows, hyperconcentrated flows and minor sheetfloods; the subaqueous fan-delta environment by debris flows, high- and low-density turbidity currents, and suspension fallout. The nearshore lacustrine facies association provides examples of deposits and processes rarely documented in lacustrine environments. High-energy oscillatory wave currents, probably related to a large fetch, reworked grains as large as 2 cm into horizontally stratified sand and gravel. Offshore-directed currents produced uncommonly large (typically 1–2 m thick) trough cross-stratified sandstone. In addition, stromatolitic carbonate interbedded with stratified coarse sandstone and conglomerate suggests a dynamic environment characterized by episodic terrigenous clastic deposition under high-energy conditions alternating with periods of carbonate precipitation under reduced energy conditions. Massive and normally graded sandstone and massive to poorly laminated mudstone characterize the offshore lacustrine facies association and record deposition by turbidity currents and suspension fallout. A depositional model constructed for the Horse Camp Formation (Member 2) precludes the existence of all four depositional environments at any particular time. Rather, phases characterized by deposition in subaerial fan, nearshore lacustrine and offshore lacustrine environments alternated with phases of subaerial fan-delta, subaqueous fan-delta and offshore lacustrine deposition. This model suggests that high-energy nearshore currents due to deep water along the lake margin reworked sediment of the fan edge, thus preventing development of a subaqueous fan-delta environment and promoting development of a well-defined nearshore lacustrine environment. Low-energy nearshore currents induced by shallow water along the     

Mid-Cretaceous basin margin carbonates, east-central Mexico

Isolated, high relief carbonate platforms developed in the intracratonic basin of east-central Mexico during Albian-Cenomanian time. Relief on the platforms was of the order of 1000 m and slopes were as steep as 20–43°. Basin-margin debris aprons adjacent to the platforms comprise the Tamabra Formation. In the Sierra Madre Oriental, at the eastern margin of the Valles-San Luis Potosi Platform, an exceptionally thick (1380m) progradational basin to platform sequence of the Tamabra Formation can be divided into six lithological units. Basinal carbonate deposition that preceded deposition of the Tamabra Formation was emphatically punctuated by an allochthonous reef block 1 km long by 0·5 km wide with a stratigraphic thickness of 95 m. It is encased in Tamabra Formation unit A, approximately 360 m of peloidal-skeletal wackestone and lithoclastic-skeletal packstone that includes some graded beds. Unit B is 73 m of massive dolomite with sparse skeletal fragments and intraclasts. Unit C, 114m thick, consists of structureless skeletal wackestone passing upward into graded skeletal packstone. Interlaminated lime mudstone and fine grained bioclastic packstone with prominent horizontal burrows are interspersed near the top. Unit D is 126 m of breccia with finely interbedded skeletal grainstone and burrowed or laminated mudstone. The breccias contain a spectrum of platform-derived lithoclasts and basinal intraclasts, up to 10 m in size. The breccias are typically grain supported (rudstone) with a matrix of lightly to completely dolomitized mudstone or skeletal debris. Beds are up to several metres thick. Unit E is 206 m of massive, sucrosic dolomite that replaced breccias. Unit F is approximately 500 m of thick bedded to massive skeletal packstone with abundant rudists and a few mudstone intraclasts. Metre scale laminated lime mudstone beds are interspersed. The section is capped by El Abra Formation platform margin limestone, consisting of massive beds of caprinid packstone and grainstone with many whole valves. Depositional processes within this sequence shift from basinal pelagic or peri-platform sedimentation to distal, platform-derived, muddy turbidity currents with a large slump block (Unit A); through more proximal (coarser and cleaner) turbidity currents (Unit B?, C); to debris flows incorporating platform margin and slope debris (Units D, E). Finally, a talus of coarse, reef-derived bioclasts (Unit F) accumulated as the platform margin prograded over the slope sequence. Interspersed basinal deposits evolved gradually from largely pelagic to include influxes of dilute turbidity currents. Units containing turbidites with platform-derived bioclasts reflect flooding of the adjacent platform. Breccia blocks and lithoclasts were probably generated by erosion and collapse of the platform during lowstands. Laminated, black, pelagic carbonates, locally cherty, are interbedded with both breccias and turbidites. At least those interbedded with turbidites may have been deposited within an expanded mid-water oxygen minimum zone during relative highstands of sea level. They are in part coeval with mid-Cretaceous black shales of the Atlantic Ocean.     

伊拉克哈法亚油田白垩系Mishrif组碳酸盐岩孔隙结构及控制因素

伊拉克哈法亚油田Mishrif组碳酸盐岩储层孔隙结构类型的差异性及对其控制因素的认识是制约该类储层分类评价的一个关键问题。综合利用岩芯、铸体薄片、扫描电镜、常规物性、压汞分析等手段及统计分析方法对储层的主要孔隙类型、喉道类型、喉道分布特征进行了研究,确定了孔隙结构划分依据和方案,并划分出了中低孔超低渗细喉型、中低孔低渗细喉型、中高孔中低渗中细喉型、高孔中低渗细喉型、中高孔中低渗细喉型、中高孔中低渗中喉型六种孔隙结构类型,并通过毛管压力曲线的形态划分出了Ⅰ-Ⅴ五类线型;研究区主要发育开阔台地相,并划分出8种储集岩类型,分别是泥晶灰岩、生屑粒泥灰岩、生屑泥粒灰岩、砂屑生屑泥粒灰岩、生屑颗粒灰岩、砂屑生屑颗粒灰岩、介壳类漂浮岩和岩溶建造岩,其中岩溶建造岩储层具有较特殊的网络状孔隙结构。因此,可以认为哈法亚Mishrif组储层主要受早成岩期岩溶作用影响,其早成岩期溶蚀具有明显的相控特征,是孔隙结构差异性的主要控制因素,极大的改善了该套储层的物性。