A review of the origin and setting of tepees and their associated fabrics

Carbonate hardgrounds often occur at the surface of shallow subtidal to supratidal, lacustrine, and subaerial carbonate shelf sediments. These are commonly disrupted and brecciated when the surface area of these crusts increases. In the subtidal environment, megapolygons form when cementation of the matrix causes the surface area of the hardgrounds to expand. Similar megapolygons form in the supratidal, lacustrine and subaerial settings when repeated incremental fracturing and fracture fill by sediment and/or cement also causes the area of the hardgrounds to expand. The arched up antiform margins of expansion megapolygons are known as tepees. The types of tepees found in the geological record include: (1) Submarine tepees which form in shallow carbonate-saturated waters where fractured and bedded marine grainstones are bound by isopachous marine-phreatic acicular and micritic cements. The surfaces of these brecciated crusts have undergone diagenesis and are bored. Unlike tepees listed below they contain no vadose pisolites or gravity cements; (2) Peritidal and lacustrine tepees are formed of crusts characterized by fenestral. pisolitic and laminar algal fabrics. This similarity in fabric makes these tepees of different origins difficult to separate. Peritidal tepees occur where the marine phreatic lens is close to the sediment surface and the climate is tropical. They are associated with fractured and bedded tidal flat carbonates. Their fracture fills contain geopetal asymmetric travertines of marine-vadose origin and/or marine phreatic travertines and/or Terra rossa sediments. The senile form of these peritidal tepees are cut by labyrinthic dissolution cavities filled by the same material. Lacustrine tepees form in the margins of shallow salinas where periodic groundwater resurgence is common. They include groundwater tepees which form over evaporitic ‘boxwork’ carbonates, and extrusion tepees which also form where periodic groundwater resurgence occurs at the margins of shallow salinas, but the dominant sediment type is carbonate mud. These latter tepee crusts are coated and crosscut by laminated micrite; the laminae extend from the fractures downward into the underlying dolomitic micrite below the crust. Both peritidal and lacustrine tepees form where crusts experience alternating phreatic and vadose conditions, in time intervals of days to years. Cement morphologies reflect this and the crusts often contain gravitational, meniscus vadose cements as well as phreatic isopachous cement rinds. (3) Caliche tepees which are developed within soil profiles in a continental setting. They are formed by laminar crusts which contain pisolites, and fractures filled by micritic laminae, microspar, spar and Terra rossa. Most of the cements are gravitational and/or meniscoid. In ancient carbonates, when their cementation and diagenetic fabric can be interpreted, tepee structures can be used as environmental indicators. They can also be used to trace the evolution of the depositional and hydrological setting.     

The Holocene non-tropical coastal and shelf carbonate province of southern Australia

Carbonate-dominant sediments are currently forming and accumulating over the extensive marine shelf of the passive margin of southern Australia. A dearth of continental detritus results from both a very low relief and a predominantly arid climate. The wide continental shelf is bathed by cold upwelling ocean waters that support luxuriant growths of bryozoans and coralline algae, together with sponges, molluscs, asteroids, benthic and some planktonic foraminifera. The open ocean coast is battered by a persistent southwest swell, resulting in erosion of calcrete-encrusted Pleistocene eolianites. Much sediment is reworked and overall shelf sedimentation rates are low. High-energy microtidal beach/dune systems occur between headlands and along the very long ocean beach in the Coorong region. The northern, more arid coastal areas also contain saline lakes that precipitate gypsum from infiltrated sea water, and display marginal facies of aragonite boxwork to fenestral carbonate crusts, with stromatolites and tepee structures. In contrast, the southern, seasonally humid Coorong region, has a predominantly continental groundwater regime where sulphate is rare, and the high summer evaporation precipitates dolomite, magnesite and aragonite muds. Fenestral crusts, breccias, tepees and some stromatolites are also present.

St. Vincent and Spencer gulfs both afford some protection from ocean swell, but tidal amplitude and currents increase, and a depth and inundation-related zonation of plants and animals is established. Muddy carbonate sand accumulates on the sea floor below 30 m, where filter-feeding bryozoans, bivalves and sponges dominate. In shallower regions, seagrass meadows contain a rich fauna that results in rapid accumulation of an unsorted muddy bioclastic sand. Mangrove woodlands backed by saline marsh with cyanobacterial mats are common, and accumulate mud-rich and gastropod-bearing sediment. As tidal amplitude and desiccation increase northward into both gulfs, a supratidal zone bare of vegetation (sabkha) becomes the site for deposition of gypsum-rich and fenestral calcitic mud.     

Association of tepees and palaeokarst in the Ladinian Calcare Rosso (Southern Alps, Italy)

The Ladinian Calcare Rosso of the Southern Alps provides a rare opportunity to examine the temporal relationships between tepees and palaeokarst. This unit comprises peritidal strata pervasively deformed into tepees, repeatedly capped by palaeokarst surfaces mantled by terra rossa. Palaeokarsts, characterized by a regional distribution across the Southern Alps, occur at the base and at the top of the unit. Local palaeokarsts, confined to this part of the platform, occur within the Calcare Rosso and strongly affected depositional facies. Tepee deformation ranges from simple antiformal structures (peritidal tepees) to composite breccias floating in synsedimentary cements and internal sediments (senile tepees). Peritidal tepees commonly occur at the top of one peritidal cycle, in association with subaerial exposure at the cycle top, while senile tepees affect several peritidal cycles, and are always capped by a palaeokarst surface. Cements and internal sediments form up to 80% of the total rock volume of senile tepees. The paragenesis of senile tepees is extremely complex and records several, superimposed episodes of dissolution, cement precipitation (fibrous cements, laminated crusts, mega-rays) and deposition of internal sediments (marine sediment and terra rossa). Petrographical observations and stable isotope geochemistry indicate that cements associated with senile tepees precipitated in a coastal karstic environment under frequently changing conditions, ranging from marine to meteoric, and were altered soon after precipitation in the presence of either meteoric or mixed marine/meteoric waters. Stable isotope data for the cements and the host rock show the influence of meteoric water (average δ¹⁸O= - 5·8‰), while strontium isotopes (average ⁸⁷Sr/86Sr=0·707891) indicate that cements were precipitated and altered in the presence of marine Triassic waters. Field relationships, sedimentological associations and paragenetic sequences document that formation of senile tepees was coeval with karsting. Senile tepees formed in a karst-dominated environment in the presence of extensive meteoric water circulation, in contrast to previous interpretations that tepees formed in arid environments, under the influence of vadose diagenesis. Tepees initiated in a peritidal setting when subaerial exposure led to the formation of sheet cracks and up-buckling of strata. This porosity acted as a later conduit for either meteoric or mixed marine/meteoric fluids, when a karst system developed in association with prolonged subaerial exposure. Relative sea level variations, inducing changes in the water table, played a key role in exposing the peritidal cycles to marine, mixed marine/meteoric and meteoric diagenetic environments leading to the formation of senile tepees. The formation and preservation in the stratigraphic record of vertically stacked senile tepees implies that they formed during an overall period of transgression, punctuated by different orders of sea level variations, which allowed formation and later freezing of the cave infills.     

Cavity‐filling dolomite speleothems and submarine cements in the Ediacaran Dengying microbialites,South China: Responses to high‐frequency sea‐level fluctuations in an ‘aragonite–dolomite sea’

Speleothems, mostly composed of calcium carbonate, are widely present in modern karst‐originated caves, but have rarely been reported in palaeokarst systems. This study presents a novel type of dolomite speleothem and subsequent submarine dolomite cement, which are widely present in the upper Ediacaran Dengying Formation in the upper Yangtze area. These precipitated materials occur in the cavity system that cuts across several peritidal cycles. The interconnected cavity networks with irregular shapes, embayed walls, internal breccias on cavity floor and their preferential development in the shallower cycle tops (for example, tepee‐deformed beds) suggest that they were initially generated by subaerial dissolution. As the earliest infills, the hemispherical protrusions, icicle‐like pendants and ground‐up columns show similar morphological features and occurrence patterns to the cave popcorn, stalactites and stalagmites, respectively. Thus, these earliest infills are speleothems resulting from associated meteoric precipitation during subaerial exposure. The isopachous growth pattern of subsequent more extensive fibrous dolomite cements points to a submarine diagenetic environment in which they were precipitated. Microscopically, the micritic to micro‐crystalline dolomite, acicular dolomite in speleothems and the subsequent fibrous dolomite share similar crystal fabrics to metastable precursors (for example, Mg‐calcite). Meanwhile, the carbon‐oxygen isotope compositions of the speleothem and fibrous dolomite, although partly altered by burial diagenesis, share a large overlap with host rock and coeval marine carbonates all over the Yangtze Platform. For these reasons, these speleothems and fibrous cements are considered to have been initially precipitated as metastable carbonate precursors in meteoric and submarine environments, respectively, and stabilized during submarine mimetic dolomitization. The cyclic occurrence of cavity systems filled with speleothems and submarine cements reflects periodic subaerial exposure and marine flooding of broad tidal flat in the upper Yangtze area, driven by high‐frequency sea‐level fluctuations. Furthermore, the Neoproterozoic seawater chemistry that favoured early dolomitization of carbonate precursor mineralogies was an advantage for the preservation of fabrics from metastable precursor minerals.     

Antecedent aeolian dune topographic control on carbonate and evaporite facies: Middle Jurassic Todilto Member,Wanakah Formation,Ghost Ranch,New Mexico,USA

The Middle Jurassic Todilto Member of the Wanakah Formation is a carbonate and gypsum unit inset into the underlying aeolian Entrada Sandstone in the San Juan Basin. Field and thin section study of the uppermost Entrada and Todilto at Ghost Ranch, New Mexico, identified Todilto facies and their relationship to remnant Entrada dune topography. Results support the previous interpretation that the Entrada dunes, housed in a basin below sea level, were rapidly flooded by marine waters. Mass wasting of the dunes gave rise to sediment‐gravity flows that largely buried remnant dune topography, leaving ca 12 m of relief that defined the antecedent condition for Todilto deposition. Previously interpreted as seasonal varves deposited in a stratified water body, the Todilto is reinterpreted as a microbial biolaminite. Most diagnostic are organic‐rich laminae with structures characteristic of filamentous microbes and containing trapped aeolian silt, and clotted‐texture laminae with a fabric associated with calcification of extracellular polymeric substances. The spatial arrangement of Todilto facies is controlled by the dune palaeotopography. A continuous basal laminated mudstone thickens over the dune crest, reflecting the optimum conditions for microbial mat development, and is interpreted to have been deposited when marine waters submerged the topography. Subsequent drying caused emergence of the crestal area, and formation of tepee structures and a dissolution breccia. Gypsiferous mudflats and periodic ponds occupied the dune flanks and interdune area, with gypsum concentrated within the interdune area. Entrada sands remained unstable during Todilto deposition with common injection structures into the Todilto, and a remnant slope caused the downslope movement and folding of Todilto strata on the upper lee face. Although some expansion of the gypsum occurred in the subsurface, facies architecture fostered development of a dissolution front adjacent to the interdune gypsum body with section collapse of gypsiferous limestone on the dune flanks.     

Controls and predictability of carbonate facies architecture in a Lower Jurassic three-dimensional barrier-shoal complex (Djebel Bou Dahar, High Atlas, Morocco)

Spatial information on lithofacies from outcrops is paramount for understanding the internal dynamics, external controls and degree of predictability of the facies architecture of shallow‐water carbonate‐platform tops. To quantify the spatial distribution and vertical stacking of lithofacies within an outer‐platform shoal‐barrier complex, integrated facies analysis and digital field technologies have been applied to a high‐relief carbonate platform exposed in the Djebel Bou Dahar (Lower Jurassic, High Atlas, Morocco). The outer platform is characterized by subtidal, cross‐bedded, coarse grainstone to rudstone grading into supratidal, pisoidal packstone‐rudstone with tepees that together formed a 350 to 420 m wide shoal‐barrier belt parallel to the margin. This belt acted as a topographic high separating a restricted lagoon from the subtidal, open marine region. Low‐energy tidal flats developed on the protected flank of the barrier facing the lagoon. Lithofacies patterns were captured quantitatively from outcrop and integrated into a digital outcrop model. The outcrop model enabled rapid visualization of field data and efficient extraction of quantitative data such as widths of facies belts. In addition, the spatial heterogeneity was captured in multiple time slices, i.e. during different phases of cyclic base‐level fluctuations. In general, the lateral continuity of lithofacies is highest when relative water depth increased during flooding of the platform top, establishing low‐energy subtidal conditions across the whole platform, and when the accommodation space was filled with tidal flat facies. Heterogeneity increased during deposition of the relief‐building bar facies that promoted spatial diversification of depositional environments during the initial phases of accommodation space creation. Cycles commonly are composed of a thin transgressive tidal flat unit, followed by coated‐grain rudstone bar facies. Lateral to the bar facies, pisoidal‐grainstone beach deposits accumulated. These bar and beach deposits were overlain by subtidal lagoonal facies or would grow through the maximum flooding and highstand. There the bars either graded into supratidal pisoidal facies with tepees (when accommodation space was filled) or were capped by subaerial exposure (due to a sea‐level fall). Modified embedded Markov analysis was used to test the presence of common ordering in vertical lithofacies stacking in a stationary interval (constant depositional mode). Analysis of individual sections did not reveal any ordering, which may be related to the limited thickness of these sections. Composite sections, however, rejected the null hypothesis of randomness. The addition of stratigraphically significant information to the Markov analysis, such as exposure surfaces and lateral dimensions of facies bodies, strengthens the verdict of unambiguous preferential ordering. Through careful quantitative reconstruction of stratal geometry and facies relationships in fully integrated digital outcrop models, accurate depositional models could be established that enhanced the predictability of carbonate sediment accumulation.     

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     

Tepee-shaped agglutinated microbialites: an example from a Famennian carbonate platform on the Lennard Shelf, northern Canning Basin, Western Australia

Distinctive, metre‐scale antiformal structures are well developed in a Famennian carbonate platform in the Chedda Cliffs area of the Lennard Shelf reef complexes. The structures are distinguished by chevron‐shaped crests and thickened cores and contain abundant non‐skeletal allochems (ooids/pisoids, peloids and intraclasts) of silt to pebble size and variably developed laminations and fenestrae. The internal morphology and pervasive occurrence of fenestral clotted and wavy laminated fabrics suggest that these structures are microbial mounds composed of agglutinated stromatolites and thrombolites. Microbial fabrics most probably originated through sediment trapping and binding by microbial mats with early lithification involving microbial calcification and cementation of trapped sediment. The facies and stratigraphic context of the mounds support a shallow subtidal, transitional backreef to reef‐flat setting; however, alone these mounds do not provide unequivocal environmental information. Other large antiformal structures in Famennian platforms on the Lennard Shelf, previously described as tepee structures, show morphological similarities to the Chedda Cliffs mounds, which suggests that these other structures may also be microbial mounds. The presence of microbial mounds in platform successions further highlights the importance of microbial communities in the Lennard Shelf reef complexes.     

东营凹陷泥页岩矿物组成及脆度分析

对东营凹陷泥页岩矿物组成、脆度进行了系统研究,并与北美页岩进行了对比。研究表明:东营凹陷沙三下和沙四上亚段泥页岩主要由黏土矿物、陆源碎屑矿物（主要指石英和长石）和自生非黏土矿物（主要指碳酸盐和黄铁矿）三部分组成;泥页岩黏土矿物含量平均只有25％左右,远低于海相泥岩和传统观点;石英含量平均为29%,远低于海相页岩;长石含量平均约5％,高于海相泥岩;自生的碳酸盐矿物含量较高,平均约37％;分析认为,陆相盆地面积较小、距物源区近以及相变快是导致研究区泥页岩矿物组成特征的主要原因。鉴于此,提出了新的脆度计算方法并计算了东营凹陷泥页岩的石英脆度、碳酸盐脆度和总脆度。对比发现,碳酸矿物盐含量与泥页岩脆度相关性较好,是影响泥页岩脆度的主要因素。最后指出:研究区泥页岩脆性较好,易于形成天然裂缝或被压裂,是页岩气潜在的有利储层。     

Submarine origin for the Neoproterozoic Wonoka canyons,South Australia

An examination of the deeply incised Ediacaran Wonoka canyons in the Adelaide Geosyncline (most recently interpreted as subaerial valleys) demonstrates their submarine origin, and confirms them as some of the best examples of ancient outcropping submarine canyons in the world. The entire canyon-fill succession is interpreted to be of deep-water (below wave base) origin, consisting of calcareous shale and siltstone together with a variety of mass-flow deposits including turbidites, grain flows and debris flows. The canyon fill lacks definitive shallow-water structures (e.g. mud cracks, fenestral fabrics or wave ripples) at all stratigraphic levels. Canyon-lining carbonate crusts that have previously been interpreted as non-marine calcretes or tufas (and used to suggest a non-marine origin for the canyons) are argued to be of deep-water, marine, microbial origin. Extremely negative carbon isotope values from the canyon-fill and canyon-lining crusts have a primary marine origin. Previously interpreted deepening upward trends in the canyon fill (used as evidence of a subaerial erosion episode followed by drowning) are suggested to be fining upward trends, caused by the transition from canyon cutting to canyon filling, with the majority of the fill being of deep-water slope origin. The basal conglomeratic canyon-fill sediments represent the last vestiges of the high-energy, deep-water, canyon-erosion environment in which the incisions formed. A deep-water origin for the canyons is consistent with all previous stratigraphic observations of the Wonoka canyons, including the conspicuous lack of regional unconformities in the lower Wonoka Formation, and their emanation from the deep-water facies of the Wonoka Formation. A submarine canyon origin also removes the need for extreme (~ 1 km) relative sea level fluctuation and associated problems (i.e. an enclosed basin with Messinian-style evaporative drawdown or thermal uplift above a migrating mantle plume) required by the subaerial valley hypotheses.     

豫西晚元古代洛峪群沉积作用及环境演化

洛峪群本身构成一个完整的海侵-海退沉积序列。早期海侵使得洛峪群底部滨岸沉积逐渐被陆棚沉积所超覆,并造成典型的海绿石砂岩凝缩层。随后的海退导致明显的海岸进积,在垂向上形成外陆棚-内陆棚-滨岸/障壁岛-泻湖沉积序列。洛峪群中的硅质碎屑岩沉积主要受风暴控制,形成各种特征的丘状和洼状交错层砂岩以及浅海浊积岩。洛峪群是华北地块南缘中、晚元古代裂谷期的产物,并可能处于裂谷热沉降阶段的后期。     

Calcareous crust (caliche) profiles and early subaerial exposure of Carboniferous carbonates, northeastern Kentucky

Sequences of laminated limestones found within thin Carboniferous carbonate strata of northeastern Kentucky were studied to determine their origin and palaeo-environmental significance. These laminated zones are strikingly similar to Holocene and Pleistocene surficial calcareous crusts (caliche) profiles that occur in various parts of the world. Carboniferous laminated carbonates are associated with shallow marine carbonate units, palaeokarst, and overlying palaeosol zones. A typical laminated profile ranges in thickness from 1 to 2 m and contains brecciated, light olive-grey to brown micrite that lacks distinctive bedding. Structures and textures common in most profiles include: (1) calcareous and silicious laminae (laminae form diffuse, alternating light and dark bands that generally parallel bedding but often fill fractures and vugs within the rocks); (2) particles (allochems, and micrite and microspar fragments) coated by brown microcrystalline calcite; (3) brecciated texture; (4) circular to elliptical fossil moulds (occur in sinuous patterns and fill fractures within the rocks); (5) large and small scale fracture patterns. Subaerial weathering and vadose diagenesis of carbonate mud banks or islands is suggested as a mechanism for the formation of these Carboniferous calcareous crust profiles. These ‘crusts’ formed by a combination of solution (karsting), brecciation, and soil development that transformed an exposed marine biomicrite (‘host’ rock) into a porous subsoil rubble. Laminated ‘crusts’ and coated particles developed as the result of dissolution and reprecipitation of CaCO₃ and SiO₃ from the soil and carbonate bedrock. Carboniferous laminated carbonates in northeastern Kentucky are often referred to as ‘algal limestones’ because of their superficial similarity to some modern and ancient algal structures. This study, however, reveals numerous characteristics that can only be explained by diagenesis in a subaerial environment.     

Facies architecture of an isolated carbonate platform: tracing the cycles of the Latemàr (Middle Triassic, northern Italy)

The 720-m-thick succession of the Middle Triassic Latemàr Massif (Dolomites, Italy) was used to reconstruct the lagoonal facies architecture of a small atoll-like carbonate platform. Facies analysis of the lagoonal sediments yields a bathymetric interpretation of the lateral facies variations, which reflect a syndepositional palaeorelief. Based on tracing of lagoonal flooding surfaces, the metre-scale shallowing-upward cycles are interpreted to be of allocyclic origin. Short-term sea-level changes led to subaerial exposure of wide parts of the marginal zone, resulting in the development of a tepee belt of varying width. Occasional emergence of the entire lagoon produced lagoon-wide decimetre-thick red exposure horizons. The supratidal tepee belt in the backreef area represented the zone of maximum elevation, which circumscribed the sub- to peritidal lagoonal interior during most of the platform's development. This tepee rim, the subtidal reef and a sub- to peritidal transition zone in between stabilized the platform margin. The asymmetric width of facies belts within individual metre-scale cycles was caused by redistribution processes that reflect palaeowinds and storm paths from the present-day south and west. The overall succession shows stratigraphic changes on a scale of tens of metres from a basal subtidal unit, overlain by three tepee-rich intervals, separated by tepee-poor units composed of subtidal to peritidal facies. This stacking pattern reflects two third-order sequences during the late Anisian to early middle Ladinian.     

Anatomy and origin of toplap in a mixed carbonate-clastic system, Seven Rivers Formation (Permian, Guadalupian), Guadalupe Mountains, New Mexico, USA

The Seven Rivers Formation exposed in Slaughter Canyon, Guadalupe Mountains, New Mexico, reveals complex relations between long- and short-term relative changes in sea-level, shelf configuration and sedimentation, which interacted to create a distinct toplap geometry. At least five sandstones diverge basinward from a prominent boundary unit marking the surface of toplap at the top of the Seven Rivers Formation and create a series of prograding, shingled clinoforms. The boundary unit is a horizontal, well-sorted, quartz arenite underlain across the shelf by peritidal carbonate or by other merging sandstones. Preserved palaeotopography is indicated by facies changes downdip and the presence of horizontal geopetal indicators in inclined beds. Near the boundary unit (updip), merging sandstones contain rare sedimentary structures including evaporite moulds and irregular fenestrae and are bounded above and below by peritidal carbonate with microbial laminae, fenestral fabrics and mudcracks. Laterally (downdip), the sandstone-bounding peritidal carbonate facies pass into subtidal carbonate facies (ooid-peloid-fusulinid-dasyclad-mollusc pack- and grainstone) and interbedded sandstones contain sedimentary structures such as ripple marks and trough to planar cross-stratification, as well as ooids, fusulinids and other carbonate grains. Toplap is interpreted to have developed by sediment bypass across a subaerially exposed shelf while sedimentation continued in still-submerged areas downdip from the shelf crest, and hence represents depositional toplap. Physical tracing of subaerial exposure surfaces suggests that the shoreline migrated up and down palaeoslope several times. The vertical component of five short-term shoreline migrations decreased during formation of the toplap geometry. Sea-level rose to approximately the same position following each fall to create the toplap geometry. This depositional toplap is the stratigraphic result of high-‘frequency’ relative changes of sea-level that combined to produce the larger-scale geometry. We suggest that changing amplitudes of relative sea-level may play a significant role in the stratigraphic evolution of platforms and that separating ‘short-term’and ‘long-term’relative sea-level may be ambiguous in such instances.     

鄂尔多斯盆地延长组次生矿物与裂缝形成时限分析

鄂尔多斯盆地延长组发育大量构造裂缝,对于裂缝的形成机理、成因动力学以及与油气成藏关系都已得到了比较系统的研究,但对于裂缝中充填的次生矿物,以及次生矿物与裂缝形成时限之间的关系,尚鲜有研究。本文在次生矿物的矿化作用和成岩作用分析基础之上,应用碳氧同位素和流体包裹体分析方法,结合区域构造热事件以及盆地热演化史,对次生矿物与裂缝形成时限的约束关系进行研究。首先,通过次生矿物的镜下观察分析,获知充填次生矿物的裂缝为张性裂缝,且可判断裂缝扩展方向;根据成岩作用分析,裂缝在形成时间上早于或同期于次生矿物形成时间。其次,根据延长组储层砂岩和裂缝中碳酸盐胶结物的碳氧同位素分析,得知露头中大量被碳酸盐胶结物充填的E-W 和N-S 向裂缝,在形成时间上早于或与碳酸盐脉体同期形成。最后,结合流体包裹体和盆地埋藏热演化史分析,得知盆内地下大量的热液活动主要发生在140～100 Ma 之间（即早白垩世）,也是盆地大规模成岩作用时期。基于上述分析,充填碳酸盐胶结物的E-W 向和N-S 向裂缝组,在形成时间上,应早于或同期于充填次生矿物的形成时间,即在早白垩世或者早白垩世之前形成。     

川西南上震旦统灯影组沉积期的暴露标志及其意义

对川西南乃至整个扬子区上震旦统灯影组的研究已有约70年的历史。以往的研究成果大致可归结为:(1)根据地层的岩石及生物特征将灯影组划分为:一段(下贫藻段)、二段(富藻层和上贫藻层)、三段、顶段(麦地坪段),其中一、二、三段的年代为晚震旦世,顶段为早寒武世;(2)从沉积学研究的角度出发,认为灯影组为潮坪环境的碳酸盐台地沉积;(3)通过生物地层学的研究,确定了大量的藻类、微古植物、小壳生物门类及其属种,以及与生物生存活动有关的生物遗迹化石。以往的研究成果无疑对我们分析、认识灯影组     

Subaerial exposure and deposition of shallowing upward sequences: evidence from stable isotopes of Purbeckian peritidal carbonates (basal Cretaceous), Swiss and French Jura Mountains

Purbeckian (lowermost Cretaceous) peritidal carbonates are characterized by open marine, lagoonal, intertidal and lacustrine facies arranged in Milankovitch-type shallowing upward sequences. Shallowing upward sequences typically consist of 2–6 individual beds. The sequences may be (i) complete, (ii) incomplete or (ii) pedogenetically overprinted, reflecting the duration of subaerial exposure and/or the extent of erosion and pedogenetic modification at the cycle tops. The stable isotopic composition of the peritidal micrites reveals homogenous δ¹⁸O values attributed to diagenetic stabilization in a meteoric, water-buffered system. Carbon isotopes show three distinctly different carbon isotope patterns dependent on the completeness of the shallowing upward sequences. Complete shallowing upward sequences consist of 4–6 individual carbonate beds. The carbon isotope values show a facies-dependent pattern: open marine carbonate muds record enriched δ¹³C values of +0·28‰ while lagoonal (−0·82‰), intertidal (−2·46‰) and lacustrine micrites (−2·96‰) are increasingly depleted. This distinct pattern is explained by carbonate mud deposition in environments of differing salinity and marine influence. Incomplete sequences (2–5 carbonate beds) are characterized by depleted δ¹³C values below subaerial exposure surfaces that become progressively enriched in ¹³C with increasing depth. Pedogenetically overprinted sequences (1–3 carbonate beds) show strong ¹³C depletion throughout the sequence with little variation in the carbon isotopic composition. The depleted values (−4·5‰) of the pedogenetically altered micrites suggest that modification during subaerial exposure was associated with equilibration with meteoric solutions enriched in isotopically light soil gas CO₂. The duration of subaerial exposure is the most crucial factor determining the extent of pedogenetic alterations, the completeness of the shallowing upward sequences and the carbon isotope pattern. The recorded patterns clearly illustrate that micrites have a good potential for the preservation of their primary carbon isotopic composition if the duration of subaerial exposure is rather brief. Otherwise, the recorded carbon isotope patterns may support sequence stratigraphic analysis by providing a refinement of the time-stratigraphic interpretation.     

柴达木盆地西部北区新近系非常规低渗储集层特征及控制因素分析

柴达木盆地西部北区新近系储集层岩石成分复杂,矿物成分包括石英、长石等陆源碎屑,也有较多碳酸盐矿物和黏土矿物,且粒度偏细,通过一般薄片鉴定难以准确进行岩性定名。本文主要对该区南翼山、油泉子、小梁山和咸水泉四个含油构造储集层岩石类型进行分析,基于岩芯观察和薄片鉴定,借助X射线全岩矿物分析,认为该区储集层岩石为陆源碎屑物质和化学成因的碳酸盐混合沉积的岩石类型,按黏土含量、碳酸盐含量和石英长石含量做出矿物三角图,提出岩性分类方案,将储集层岩性划分为泥岩、砂岩、碳酸盐岩和混合沉积类。该区储集层岩石物性普遍较差,孔隙类型包括粒间溶蚀孔隙、微裂缝和微孔隙等。压实作用、溶蚀作用及胶结充填作用是控制储集层性能的主要因素。     

Rare earth elements of seep carbonates: Indication for redox variations and microbiological processes at modern seep sites

At marine seeps, methane is microbially oxidized resulting in the precipitation of carbonates close to the seafloor. Methane oxidation leads to sulfate depletion in sediment pore water, which induces a change in redox conditions. Rare earth element (REE) patterns of authigenic carbonate phases collected from modern seeps of the Gulf of Mexico, the Black Sea, and the Congo Fan were analyzed. Different carbonate minerals including aragonite and calcite with different crystal habits have been selected for analysis. Total REE content (ΣREE) of seep carbonates varies widely, from 0.1 ppm to 42.5 ppm, but a common trend is that the ΣREE in microcrystalline phases is higher than that of the associated later phases including micospar, sparite and blocky cement, suggesting that ΣREE may be a function of diagenesis. The shale-normalized REE patterns of the seep carbonates often show different Ce anomalies even in samples from a specific site, suggesting that the formation conditions of seep carbonates are variable and complex. Overall, our results show that apart from anoxic, oxic conditions are at least temporarily common in seep environments.