Study of the Messinian carbonate-evaporite lithofacies offshore Western Libya

This study represents a preliminary investigation of the late Messinian subsurface Marsa Zouaghah Formation in the Western Libyan Offshore, Central Mediterranean Sea. The formation was deposited in three major environmental settings: (a) Marginal Sabkha; (b) Open Lagoon; and (c) Hypersaline Lagoon. The marginal sabkha and open lagoonal settings are locally interrupted by intertidal oolitic shoal deposits. The marginal sabkha facies is replaced in central parts of the Libyan offshore by a narrow zone of aeolian-fluvial facies, the distribution and thickness of which is entirely controlled by a local uplift, the Tripoli Nose'. The marginal sabkha facies broadly defines the late Messinian palaeoshoreline lying parallel to, and north of, the E-W Jifarah fault system which dominated the southern part of the Libyan offshore. This fades is, thus, interpreted as being structurally controlled by fault systems. The hypersaline lagoonal facies is developed in areas of relatively higher rates of subsidence than that of adjacent facies belts. It is therefore, related to restriction formed by continuous subsidence and evaporation. The Marsa Zouaghah Formation constitutes saltern evaporates deposited over a wide platform in sabkha and lagoonal settings, forming part of the 'basin-wide-evaporites' of the Mediterranean Basin deposited during the Messinian salinity crisis. The evaporites formed during a major relative sea-level fall within a subsiding basin situated on the northern continental margin of the African plate. Local, vertical and lateral variations in lithofacies and thickness within the Messinian deposits of the north-west Libyan offshore were controlled by contemporaneous strike-slip movements in addition to sea-level change.     

Sedimentation, distribution and diagenesis of organic matter in a recent carbonate environment, Abu Dhabi, U.A.E.

In the modern hypersaline carbonate lagoon and sabkha sedimentary environments of Abu Dhabi (United Arab Emirates), three types of organic matter originate respectively from microbial mat, Avicennia mangrove, and Halodule lagoonal seagrass. The study of recent sedimentary processes and cross sections through the sabkha sediments lead to the definition of organo-sedimentary facies based on geochemical and sedimentological criteria. This permits the construction of an organo-sedimentary sequence which expresses the Holocene sedimentary record involving a transgressive and a regressive sequence. The various organic facies occur in both sequences.Heterogeneity within the individual organic facies reflects several factors, including sedimentation dynamics, mineral matrix, oxidation and reduction, and selective organic and mineral diagenesis. These parameters are discussed in terms of depositional environment and location within the organo-sedimentary sequence. Changes in distribution, quantity, and preservation potential of the buried organic matter are discussed in terms of sea level changes and sedimentary accretion rates.     

Distinction between the Storegga tsunami and the holocene marine transgression in coastal basin deposits of western Norway

Many coastal lakes were inundated by both the Storegga tsunami (7000 ¹⁴C yr BP) and the mid-Holocene sea-level rise (the Tapes transgression) in western Norway. The tsunami eroded lake bottoms and deposited graded and/or massive beds of sand, rip-up clasts, and coarse plant material. By contrast, when the rising sea entered the lakes, it deposited only gyttja, silt and fine sand, without causing much erosion of the underlying lake sediments. Storegga tsunami deposits in some coastal lakes were interpreted previously as ordinary marine sediments from the Tapes transgression. Our reinterpretation of these deposits shows that the transgression maximum phase was reached after 6500 yr BP, more than 1000 yr later than previously inferred for the coast of Sunnmøre. The new data cannot be combined in a shoreline diagram without showing the 6000 yr BP and 7000 yr BP shorelines as slightly warped. © 1998 John Wiley & Sons, Ltd.     

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.     

Are carbonate barrier islands mobile? Insights from a mid to late-Holocene system,Al Ruwais,northern Qatar

Barrier islands are important landforms in many coastal systems around the globe. Studies of modern barrier island systems are mostly limited to those of siliciclastic realms, where the islands are recognized as mobile features that form on transgressive coastlines and migrate landward as sea-level rises. Barrier islands of the ‘Great Pearl Bank’ along the United Arab Emirates coast are the best-known carbonate examples. These Holocene islands, however, are interpreted to be anchored by older deposits and immobile. The mid-Holocene to late-Holocene depositional system at Al Ruwais, northern Qatar, provides an example of a mobile carbonate barrier island system, perhaps more similar to siliciclastic equivalents. Sedimentological and petrographic analyses, as well as ¹⁴C-dating of shells and biogenic remains from vibracored sediments and surface deposits, show that after 7000 years ago a barrier system with a narrow back-barrier lagoon formed along what is now an exposed coastal zone, while, contemporaneously, a laterally-extensive coral reef was forming immediately offshore. After 1400 years ago the barrier system was forced to step ca 3 km seaward in response to a sea-level fall of less than 2 m, where it re-established itself directly on the mid-Holocene reef. Since that time, the barrier has retreated landward as much as 1000 m to its current position, exposing previously-deposited back-barrier lagoonal sediment at the open-coast shoreline. In modern neritic warm-water carbonate settings mobile barrier island systems are rare. Their construction and migration may be inhibited by reef formation, early cementation, and the relative inefficiency of sourcing beach sediments from open carbonate shelves. Carbonate barrier island systems likely formed more commonly during geological periods when ramps and unrimmed shelves predominated and in calcite seas, when meteoric cementation was minimized as a result of initial calcitic allochem mineralogy. As with their siliciclastic analogues, however, recognition of the influence of these transient landforms in the rock record is challenging.     

Reef geometries, erosion surfaces and high-frequency sea-level changes, upper Miocene Reef Complex, Mallorca, Spain

The upper Miocene Reef Complex of Mallorca is a 20-km prograding unit which crops out in sea cliffs along the southern side of the island. These vertical and exceptionally clean outcrops permit: (i) identification of different facies (lagoon, reef front, reef slope and open platform) and their geometries and boundaries at different scales, ranging from metre to kilometre, and (ii) construction of a 6-km-long high-resolution cross-section in the direction of reef progradation. This cross-section shows vertical shifts of the reefal facies and erosion surfaces linked to a general progradational pattern that defines the accretional units. Four hierarchical orders of magnitude (1-M to 4-M) of accretional units are identified by consideration of the vertical facies shifts and by which erosion surfaces are truncated by other erosion surfaces. All these orders show similar patterns: horizontal beds of lagoonal facies in the upper part (landward), reefal and slope facies with sigmoidal bedding in the central part, and open-platform facies with subhorizontal bedding in the lower part (basinwards). The boundaries are erosion surfaces, horizontal over the lagoon facies, dipping basinwards over the reef-front facies and connecting basinwards with their correlative conformities over the reef-slope and open-platform facies. The four orders of accretional units are interpreted in terms of four (1-M to 4-M) hierarchies of sea-level cycles because (i) there is a close relation between the coral growth and the sea surface, (ii) there are vertical shifts in the reefal facies and their relation to the erosion surfaces, and (iii) there was very little tectonic subsidence in the study area during the late Miocene. Additionally, all these units can be described in terms of their position relative to the sea-level cycle: (i) the reefs prograde on the open-platform sediments during low stands of sea-level; (ii) aggradation of the lagoon, reef and open-platform facies dominates during sea-level rises, and the lagoonal beds onlap landwards upon the previous erosion surface; (iii) reefal progradation occurs during high stands of sea-level; and (iv) the 2-M sea-level fall produces an off-lapping reef and there is progradation with downward shifts of the reefal facies and erosion landward on the emerged (older) reefal units (A-erosion surfaces); the 3-M and 4-M sea-level falls produce only erosion (B-and C-erosion surfaces). Although precise age data do not exist at present, some speculations upon the frequency of these Miocene relative sea-level cycles can be made by comparisons with Pleistocene cyclicity. There is a good correlation between the Miocene 2-M cycles and the 100-ka Pleistocene cycles. Consequently, the 1-M cycles can be assigned to a fourth order in relation to previously proposed global cycles and the 2-M to fifth-order cycles. All these accretional units could be defined as ‘sequences’, according to the definition as commonly used in sequence stratigraphy. However, they represent higher than third-order sea-level cycles, but are not parasequences. The term subsequence, therefore, is suggested to define ‘a part of a sequence bounded by erosion surfaces (mostly subaerial) and their correlative conformities basinwards'. A hierarchy of subsequences can be established.     

SABKHA CYCLES IN THE ARAB/DARB FORMATION OFF THE TRUCIAL COAST OF ARABIA

SUMMARY
Study of 176 ft. (53.7 m) of core from the Arab/Darb Formation of the Umm Shaif Field, Abu Dhabi Marine Areas, has revealed a sequence of sediments which can be related to nine distinct cycles of sabkha formation. The sabkha cycle consists of a basal algal grainstone/boundstone (which is interpreted as a shoal) passing upwards through lagoonal dolomite, intertidal algal mat and into a final supratidal development of nodular anhydrite and associated dolomite. Dolomiti-sation and the formation of nodular anhydrite were early-stage products of diagenesis. Poikilotopic anhydrite is common in the lagoonal dolomites but it is thought that this was not formed until the sediment was completely lithified.
Stylolites, although of small amplitude, are more common in dolomite than in limestone; a generalisation which is at variance with earlier observations.     

Primary dolomite in the Late Triassic Travenanzes Formation,Dolomites, Northern Italy: Facies control and possible bacterial influence

In the late Carnian (Late Triassic), a carbonate‐clastic depositional system including a distal alluvial plain, flood basin and sabkha, tidal flat and shallow carbonate lagoon was established in the Dolomites (Northern Italy). The flood basin was a muddy supratidal environment where marine carbonates and continental siliciclastics interfingered. A dolomite phase made of sub‐micrometre euhedral crystals with a mosaic microstructure of nanometre‐scale domains was identified in stromatolitic laminae of the flood basin embedded in clay. This dolomite is interpreted here as primary and has a nearly stoichiometric composition, as opposed to younger early diagenetic (not primary) dolomite phases, which are commonly calcian. This primary dolomite was shielded from later diagenetic transformation by the clay. The stable isotopic composition of dolomite was analyzed along a depositional transect. The δ¹³C values range between ca ?6‰ and +4‰, with the most ¹³C‐depleted values in dolomites of the distal alluvial plain and flood basin, and the most ¹³C‐enriched in dolomites of the tidal flat and lagoon. Uniform δ¹⁸O values ranging between 0‰ and +3‰ were found in all sedimentary facies. It is hypothesized that the primary dolomite with mosaic microstructure nucleated on extracellular polymeric substances secreted by sulphate reducing bacteria. A multi‐step process involving sabkha and reflux dolomitization led to partial replacement and overgrowth of the primary dolomite, but replacement and overgrowth were facies‐dependent. Dolomites of the landward, clay‐rich portion of the sedimentary system were only moderately overgrown during late dolomitization steps, and partly retain an isotopic signature consistent with bacterial sulphate reduction with δ¹³C as low as ?6‰. In contrast, dolomites of the marine, clay‐free part of the system were probably transformed through sabkha and reflux diagenetic processes into calcian varieties, and exhibit δ¹³C values of ca +3‰. Major shifts of δ¹³C values strictly follow the lateral migration of facies and thus mark transgressions and regressions.     

Diagenesis of a mixed siliciclastic/evaporitic sequence of the Middle Muschelkalk (Middle Triassic), the Catalan Coastal Range, NE Spain

The Middle Muschelkalk (Middle Triassic) of the Catalan Coastal Range (north-east Spain) comprises sandstone, mudstone, anhydrite and minor carbonate layers. Interbedded sandstones and mudstones which are dominant in the north-eastern parts of the basin are terminal alluvial fan deposits. South-westward in the basin, the rocks become dominated by interbedded evaporites and mudstones deposited in sabkha/mudflat environments. The diagenetic and pore water evolution patterns of the Middle Muschelkalk suggest a strong facies control. During eodiagenesis, formation of microdolomite, anhydrite, baryte, magnesite, K-feldspar and mixed-layer chlorite/smectite was favoured within and adjacent to the sabkha/mudflat facies, whereas calcite, haematite, mixed-layer illite/smectite and quartz formed mainly in the alluvial facies. Low δ¹⁸O_SMOW values for microdolomite (+23.7 to +28.4%) and K-feldspar overgrowths (+17.3 to +17.7%) suggest either low-temperature, isotopic disequilibrium or precipitation from low-¹⁸O porewaters. Low-¹⁸O waters might have developed, at least in part, during low-temperature alteration of volcanic rock fragments. During mesodiagenesis, precipitation of quartz overgrowths and coarse dolomite occurred in the alluvial sandstones, whereas recrystallization of microdolomite was dominant in the sabkha/mudflat facies. The isotopic compositions of these mesogenetic phases reflect increasing temperature during burial. Upon uplift and erosion, telogenetic calcite and trace haematite precipitated in fractures and replaced dolomite. The isotopic composition of the calcite (δ¹⁸O_SMOW=+21.5 to +25.6%o; δ¹³C= 7.7 to - 5.6%o) and presence of haematite indicate infiltration of meteoric waters.     

Shallowing-upward sequences in Purbeckian peritidal carbonates (lowermost Cretaceous, Swiss and French Jura Mountains)

Purbeckian carbonates in the Swiss and French Jura (Goldberg Formation, lower Berriasian) comprise shallow-subtidal, intertidal, supratidal, low-energy, high-energy, marine, brackish, freshwater, and hypersaline facies. These facies are arranged in small (0–2–1.5 m thick) sequences which display a dominant shallowing-upward component, and which form the fundamental units of the highly structured Purbeckian sedimentary record. Six types of small-scale sequences can be recognized. A: intertidal to supratidal overprinting of shallow lagoonal facies; B: algal-marsh sequence with frequent dolomitization; C: sabkha sequence, often associated with collapse breccia; D: tidal-flat sequence with desiccation features; E: lacustrine sequence; F: terrestrial overprinting of subtidal or intertidal facies. Episodic event deposits such as tempestites are superimposed] Thin transgressive beds which rework elements of the underlying facies are frequently found at the base of the sequences. Green marls and black pebbles are common at the top and indicate long subaerial exposure. The sequences are often incomplete, as subtidal facies may be absent, or their upper part can be eroded. Lateral facies changes are common, which is due to the very shallow and partly emergent Purbeckian platform where various depositional environments were juxtaposed. However, many sequence boundaries are well developed and can be correlated over large parts of the study area. The Purbeckian shallowing-upward sequences were generated by climatically controlled sea-level changes. Autocyclic processes occurred locally, but were overprinted by drops of sea-level affecting the entire platform. The small-scale sequences are most probably related to the 20 000-year cycle of the precession of the equinoxes. Larger sequences with usually well-developed emersion surfaces are attributed to the 100 000 and 400 000-year eccentricity cycles of the Earth's orbit. Identification and correlation of sequence boundaries makes it possible to set up a framework of isochronous surfaces (which often cut across facies boundaries), and thus to interpret in detail the palaeogeographic, sedimentological and diagenetic evolution of the Purbeckian peritidal carbonate environments.     

Sulfur isotope studies in early Archaean sediments from Isua,West Greenland: Implications for the antiquity of bacterial sulfate reduction

The sulfur contents and sulfur isotope ratios (δ³⁴S) have been measured for samples collected from the Isua area of West Greenland in an effort to place narrower limits on the time of the rise of sulfate respirers during the Precambrian.The δ³⁴S values of the Isua sediments (3.7 × 10⁹ yr old) including the various facies of the banded iron-formations have their mean values close to zero %. (CDT) (±0.5%.) with a standard deviation of less than 1%.. This comes extremely close to the respective means yielded by the presumed tuffaceous amphibolites (+ 0.3 ± 0.9%.) and by the somewhat younger, between 3.1 and 3.7 ± 10⁹ yr, basaltic Ameralik dykes of the region (+ 0.6 ± 1.1%.).In view of the regional distribution of the Isua banded iron-formation sediments, the variety of environmental conditions under which the various facies were deposited and the complete absence of isotopic evidence for sulfate reducers, in contrast to the banded iron-formations of the middle Archaean (δ³⁴Svariesfrom ?20 to +20%.), it seems most unlikely that evidence for 'sulfate reducers' existed or will be found in other sediments of Isua age.The very small spread in δ³⁴S values for the Isua sediments is interpreted as due to minor fractionation during the passage of endogenic sulfur phases to their present sites of emplacement within the sedimentary succession.     

Late-Quaternary paleoenvironmental evolution of Lesina lagoon (southern Italy) from subsurface data

Integrated sedimentological and micropaleontological (foraminifers and ostracods) analyses of two 55 m long borehole cores (S3 and S4) drilled in the subsurface of Lesina lagoon (Gargano promontory—Italy) has yielded a facies distribution characteristic of alluvial, coastal and shallow-marine sediments. Stratigraphic correlation between the two cores, based on strong similarity in facies distribution and AMS radiocarbon dates, indicates a Late Pleistocene to Holocene age of the sedimentary succession.Two main depositional sequences were deposited during the last 60-ky. These sequences display poor preservation of lowstand deposits and record two major transgressive pulses and subsequent sea-level highstands. The older sequence, unconformably overlying a pedogenized alluvial unit, consists of paralic and marine units (dated by AMS radiocarbon at about 45–50,000 years BP) that represent the landward migration of a barrier-lagoon system. These units are separated by a ravinement surface (RS1). Above these tansgressive deposits, highstand deposition is characterised by progradation of the coastal sediments.The younger sequence, overlying an unconformity of tectonic origin, is a 10 m-thick sedimentary body, consisting of fluvial channel sediments overlain by transgressive–regressive deposits of Holocene age. A ravinement surface (RS2), truncating the transgressive (lagoonal and back-barrier) deposits in core S4, indicates shoreface retreat and landward migration of the barrier/lagoon system. The overlying beach, lagoon and alluvial deposits are the result of mid-Holocene highstand sedimentation and coastal progradation.     

Late Holocene to Recent aragonite-cemented transgressive lag deposits in the Abu Dhabi lagoon and intertidal sabkha

Modern cemented intervals (beachrock, firmgrounds to hardgrounds and concretionary layers) form in the lagoon and intertidal sabkha of Abu Dhabi. Seafloor lithification actively occurs in open, current-swept channels in low-lying areas between ooid shoals, in the intertidal zone of the middle lagoon, some centimetres beneath the inner lagoonal seafloor (i.e. within the sediment column) and at the sediment surface the intertidal sabkha. The concept of ‘concretionary sub-hardgrounds’, i.e. laminar cementation of sediments formed within the sediment column beneath the shallow redox boundary, is introduced and discussed. Based on calibrated radiocarbon ages, seafloor lithification commenced during the Middle to Late Holocene (ca 9000 cal yr bp ), and proceeds to the present-day. Lithification occurs in the context of the actualistic relative sea-level rise shifting the coastline landward across the extremely low-angle carbonate ramp. The cemented intervals are interpreted as parasequence boundaries in the sense of ‘marine flooding surfaces’, but in most cases the sedimentary cover overlying the transgressive surface has not yet been deposited. Aragonite, (micritic) calcite and, less commonly, gypsum cements lithify the firmground/hardground intervals. Cements are described and placed into context with their depositional and marine diagenetic environments and characterized by means of scanning electron microscope petrography, cathodoluminescence microscopy and Raman spectroscopy. The morphology of aragonitic cements changes from needle-shaped forms in lithified decapod burrows of the outer lagoon ooidal shoals to complex columnar, lath and platy crystals in the inner lagoon. Precipitation experiments provide first tentative evidence for the parameters that induce changes in aragonite cement morphology. Data shown here shed light on ancient, formerly aragonite-cemented seafloors, now altered to diagenetic calcites, but also document the complexity of highly dynamic near coastal depositional environments.     

辽东半岛大孤山一带沟谷埋藏泥炭的硅藻组合以及 古环境和全新世最高海平面*

在辽东半岛东部的大孤山一带,面向海岸平原的沟谷埋藏泥炭十分发育。通过对11个沟谷进行钻孔调查,确认了在被切割的丘陵台地的沟谷中,埋藏泥炭广泛地发育在海拔\{2~\}10余米的不同高度; 并对钻孔样品做硅藻分析,明确了基底海拔高度在\{4~\}2m的泥炭层之下的沉积物为海生种硅藻占优势的海相层,因而取得了海水广泛侵入大洋河平原并直达沟谷深处的微体古生物方面的证据。进一步对3个代表性的小沟谷平原从沟头到沟口连续钻孔取样以及地质剖面分析,研究沟谷埋藏泥炭在横向上的分布特征表明:沟谷埋藏泥炭是全新世高海面期以后,随着海水的退却而形成的沼泽和湖沼等环境下的溺谷型泥炭; 位于沟谷中沟头位置的海拔4m左右的泥炭底部的标高,大致代表了该区全新世最高海面期海水(平均高潮线)曾达到的位置和高度,其泥炭在沟头开始形成的时期大致代表该区全新世最高海平面期; 研究区约\{6000~\}5500年前达到全新世最高海面,当时海面高度比现在约高出1.7m。     

Geochemistry and mineralogy of a recent sabkha along the coast of Sinai, Gulf of Suez

Most efforts in the study of sea-marginal sabkhas have concentrated on the Persian Gulf, but little is known about the sediments and mineralogy of sabkhas marginal to other seas. The purpose of this paper was to present some geochemical and mineralogical observations in a recent sabkha on the coast of Sinai along the Gulf of Suez. The sabkha is composed of coarse clastic sediments with marine-derived groundwater at depth of about 1 m. The general morphology, climate and water salinity of the Gulf of Suez resemble those of the Persian Gulf, despite the fact that the content of authigenic evaporites in this sabkha is more sparse. The evaporite minerals accumulated only in the upper 30–40 cm of the sabkha, below that and down to the groundwater table, there is no accumulation of evaporites. Laterally, the salinity of the groundwater in the sabkha and the concentration of evaporites in the sediments above it increase constantly with distance from the shore. In contrast to the Persian Gulf where anhydrite is a major evaporite mineral, in Belayim gypsum is the only calcium sulphate mineral in the recent sabkha. Anhydrite is found only in an old elevated sabkha where it recrystallized from gypsum. The gypsum occurs as interstitial crystal concentrations or lithified horizons almost exclusively at the depth of 20–40 cm below the sabkha surface. Above that, in the uppermost horizons, there is in situ accumulation of interstitial halite crystals. The total concentrations of gypsum and halite are almost equal in this sabkha. The sea water recharge in El Belayim is almost exclusively by seepage through the sabkha sediments and not by flooding. The groundwater under this sabkha is only slightly more saline than the Gulf water, thus, not heavy enough for extensive downward refluxing. The major hydrodynamic process must be upward migration of the brines from the groundwater, precipitating on the way gypsum and later halite with some magnesite. Since the sediments of the sabkha are too coarse to support extensive capillary movement, the brines must, therefore, migrate upwards due to ‘evaporative pumping’.     

Sedimentology and budget of a Recent carbonate mound, Florida Keys

The sedimentology of a Recent carbonate mound is investigated to further our understanding of mound building communities, surface and subsurface mound sediments, and the overall sediment budget of mounds. Nine sedimentary facies of the surface of Tavernier mound, Florida Keys are described. These sediments are composed of Neogoniolithon, Halimeda, Porites, mollusc and foraminiferal grains, and lime mud. Muds rich in aragonite and high magnesian calcite show little mineralogical variation over the mound surface. Geochemical evidence suggests that the mud is mainly formed from breakdown of codiacean algae and Thalassia blade epibionts. Production rates of the facies are established from in situ growth rate experiments and standing-crop surveys. Annual calcium carbonate production is c. 500gm^-2, intermediate between reef and other bay and lagoonal environment production rates in the Caribbean. The internal structure of the mound, studied from piston cores and sediment probes, indicates that seven facies can be identified. Five of these can be related to the present-day facies, and occur in the upper part of the mound (gravel-mound stage). The remaining two facies, characterized by molluscs and aragonite-rich muds, occur in the lower part of the mound (mud-mound stage), and are most similar to facies from typical Florida Bay mud mounds. Mangrove peats within the mound indicate former intertidal areas and C¹⁴ dates from these peats provide a time framework for mound sedimentation. The mound appears to have formed because of an initial valley in the Pleistocene surface which accumulated mud in a shallow embayment during the Holocene transgression. A sediment budget for the mound is presented which compares production rates from present-day facies with subsurface sediment masses. During the mud mound stage production rates were similar to accumulation rates and the mound was similar to the present-day mounds of Florida Bay. During the gravel mound stage (3400 yr BP-present day), conditions were more normal marine and the establishment of Porites and Neogoniolithon on the mound increased production rates 10% over accumulation rates. This excess sediment is thought to be transported off the mound to the surrounding seabed. Models are proposed which divide carbonate mounds on the basis of internal versus external sediment supply. Comparisons are made with other Recent and ancient mounds. Similarities exist between the roles of the biotic components of late Palaeozoic mounds but major differences are found when structures and early diagenesis are compared.     

Sedimentology of a Proterozoic erg: the Venkatpur Sandstone, Pranhita-Godavari Valley, south India

Reappraisal of the Late Proterozoic Venkatpur Sandstone indicates that the bulk of the sandstone is aeolian in origin. Aeolian stratification types, namely (i) inverse graded translatent strata, (ii) adhesion laminae, (iii) grainflow strata and (iv) grainfall strata, are present throughout the outcrop belt. Nine facies have been identified that represent both aeolian and related aqueous environments within a well-developed erg. Cosets of large cross-beds at the Bellampalli section in the NW of the study area record dune fields in the interior of the sand sea. To the SE, at the Godavari River and Ramgundam sections, a progressive increase in the relative proportion of the flat-bedded to cross-bedded facies and intercalated non-aeolian facies delineates the transition from the dune-field to sand-sheet environment. An alternating sequence of aeolian and marine sediments at Laknavaram, in the extreme SE, marks the termination of the sand sea. Palaeocurrent data suggest that the NW-SE trend of the sections represents a transect across the sand sea in a direction normal to the resultant primary palaeowind direction. Abundant horizontally stratified units in the Vankatpur Sandstone do not always represent the interdune sediments. On the basis of the thickness and geometry of the units, nature of bounding surfaces and associated facies sequence, the facies is variously interpreted to represent interdune, inland sabkha, sand sheet and coastal sand flat deposits.     

Hydrology of the coastal sabkhas of Abu Dhabi, United Arab Emirates

Water fluxes were estimated and a water budget developed for the land surface and a surficial 10-m-deep section of the coastal sabkhas that extend from the city of Abu Dhabi, United Arab Emirates, west to the border with Saudi Arabia. The fluxes were estimated on the basis of water levels and hydraulic conductivities measured in wells and evaporation rates measured with a humidity chamber. In contrast with conceptual models proposed in earlier studies, groundwater inflow is estimated to be small, whereas the largest components of the water budget are recharge from rainfall and evaporation from the water table. Estimates within a rectilinear volume of sabkha, defined as 1 m wide by 10 km long by 10 m deep, indicate that about 1 m³/year of water enters and exits by lateral groundwater flow; 40–50 m³/year enters by upward leakage; and 640 m³/year enters by recharge from rainfall. Based on the water and solute fluxes estimated for the upward leakage into the sabkha, 7–8 pore volumes of brine have entered the sabkha from below since the time the sabkha became saturated (7,000 years ago) as a result of the last global sea-level rise.

Pronounced occurrence of long-chain alkenones and dinosterol in a 25,000-year lipid molecular fossil record from Lake Titicaca, South America

Our analysis of lipid molecular fossils from a Lake Titicaca (16° S, 69° W) sediment core reveals distinct changes in the ecology of the lake over an ∼25,000-yr period spanning latest Pleistocene to late Holocene time. Previous investigations have shown that over this time period Lake Titicaca was subject to large changes in lake level in response to regional climatic variability. Our results indicate that lake algal populations were greatly affected by the changing physical and chemical conditions in Lake Titicaca. Hydrocarbons are characterized by a combination of odd-numbered, mid- to long-chain (C₂₁-C₃₁) normal alkanes and alkenes. During periods when lake level was higher (latest Pleistocene, early Holocene, and late Holocene), the C₂₁n-alkane, and the C₂₅ and C₂₇ alkenes dominate hydrocarbon distributions and indicate contribution from an algal source, potentially the freshwater alga Botryococcus braunii. The C₃₀ 4 α-methyl sterol (dinosterol) increases sharply during the mid-Holocene, suggesting a greatly increased dinoflagellate presence at that time. Long-chain alkenones (LCAs) become significant during the early Holocene and are highly abundant in mid-Holocene samples. There are relatively few published records of LCA detection in lake sediments but their occurrence is geographically widespread (Antarctica, Asia, Europe, North America). Lake Titicaca represents the first South American lake and the first low-latitude lake in which LCAs have been reported. LCA abundance and distribution may be related to the temperature-dependent response of an unidentified algal precursor. Although the LCA unsaturation indices cannot be used to determine absolute Lake Titicaca temperatures, we suspect that the published LCA U₃₇^K unsaturation calibrations can be applied to infer relative temperatures for early to mid-Holocene time when LCA concentrations are high. Using these criteria, the U₃₇^K unsaturation indices suggest relatively warmer temperatures in the mid-Holocene. In contrast to previous speculation, lipid analysis provides little evidence of a greatly increased presence of aquatic plants during the mid-Holocene. Instead, it appears that a few algal species were dominant in the lake. Based on the dramatic rise in abundances of LCAs and dinosterol during the early to mid-Holocene, we suspect that the algal producers of these compounds rose in response to a combination of physical and chemical changes in the lake. These include temperature, salinity, and alkalinity changes that occurred as lake level dropped sharply during a multi-millennial drought affecting the Central Andean Altiplano.     

Formation of mud clast breccias and the process of sedimentary autobrecciation in the hominin-bearing (Homo naledi) Rising Star Cave system,South Africa

Unconsolidated mud clast breccia facies in the hominin-bearing (Homo naledi) Rising Star Cave, Cradle of Humankind, South Africa, are interpreted to have formed through a process termed sedimentary autobrecciation in this study. This process, by which most of the angular mud clast breccia deposits are thought to have formed autochthonously to para-autochthonously via a combination of erosion, desiccation, diagenesis and microbial alteration of laminated mud deposits, is thought to have taken place under relatively dry (i.e. non-flooded) conditions inside the cave. Subsequently, gravitational slumping and collapse was the dominant mechanism that produced the mud clast breccia deposits, which commonly accumulate into debris aprons. The mud clast breccia is typically associated with (micro) mammal fossils and is a common facies throughout the cave system, occurring in lithified and unlithified form. This facies has not been described from other cave localities in the Cradle of Humankind. Additionally, sedimentary autobrecciation took place during the deposition of some of the fossils within the Rising Star Cave, including the abundant Homo naledi skeletal remains found in the Dinaledi Subsystem. Reworking of the mud clast breccia deposits occurs in some chambers as they slump towards floor drains, resulting in the repositioning of fossils embedded in the breccias as evidenced by cross-cutting manganese staining lines on some Homo naledi fossil remains. The formation of the unlithified mud clast breccia deposits is a slow process, with first order formation rates estimated to be ca 8 × 10⁻⁴ mm year⁻¹. The slow formation of the unlithified mud clast breccia facies sediments and lack of laminated mud facies within these deposits, indicates that conditions in the Dinaledi Chamber were probably stable and dry for at least the last ca 300 ka, meaning that this study excludes Homo naledi being actively transported by fluvial mechanisms during the time their remains entered the cave.