Periods Of Subaerial Exposure Research Articles

Dolomitized calcrete in the Middle Devonian Winnipegosis carbonate mounds, subsurface of south–central Saskatchewan, Canada

Dolomitized calcrete is a widespread diagenetic feature in the uppermost portions of the Middle Devonian Winnipegosis mud mounds in the subsurface of south–central Saskatchewan. Individual calcrete profiles are composed of 2 to 5 horizons, including (in descending order) laminar crust, massive, pisolitic, breccia, chalky and transitional horizons. The calcrete profiles are interpreted to be pedogenic in origin, formed by diagenetic alteration of the host carbonate deposits during subaerial exposure of the mud mounds. This interpretation is based on the occurrence of an orderly set of well-differentiated horizons, disconformities, vadose pisoids, micritic stringers, circumgranular cracks, linkage coatings, and gradational contacts between the calcrete and underlying host carbonate rocks. Lack of root-related structures, such as rhizoconcretion and alveolar–septal fabric in the Winnipegosis calcrete, suggests a limited influence of macrophytes on the formation of calcrete. The occurrence of up to three discrete pedogenic calcrete profiles in a single Winnipegosis calcrete succession indicates the mounds underwent three periods of subaerial exposure that resulted from drops of water level in the Elk Point Basin of Saskatchewan during Middle Devonian time.

Day–night variation of intertidal flat sediment properties in relation to sediment stability

The majority of investigations that have measured sediment properties related to intertidal sediment stability have been undertaken during daylight subaerial exposure periods. As a consequence, models based upon such data represent only partially the intertidal flat surface conditions within any 24 h period. In this contribution, a comparison is made between surface sediment properties related to sediment stability measured during six consecutive (day/night), semi-diurnal subaerial exposure periods, at three stations on an intertidal sand flat in late March 1999. The study site was selected on the basis of its suitability for sampling and data collection at night, with special regard to safety and logistics. Seawater temperatures ranged from 4.1 to 9.6 °C, and salinities from 33.9 to 34.8. Eleven parameters related to intertidal flat sediment stability were measured, or derived. These variables included the critical erosion shear stress ( τ c), chlorophyll a, phaeopigment, and colloidal carbohydrate content, mean grain size and settling velocity of the surface (0–1 mm) sediment fraction. Bed elevation was described using an acretion/erosion parameter (AEP) (West and West 1991), whilst additional physical terms included ambient seawater salinity and temperature, as well as tidal range and wind speed, during the preceding immersion periods. One-way ANOVA was used to detect significant differences between day- and night-time emersion periods; similarly, principal components analysis (PCA) was applied to detect continuous variation between properties. The results show a high degree of temporal and spatial variability between day- and night-time intertidal flat variables, the PCA differentiating clearly between day and night conditions. Surface sediments across the intertidal flat exhibited varying degrees of biostabilisation. The maximum biostabilisation coefficient (18) was recorded at night in high microalgal biomass areas; the minimum (5) occurred during both day and night, in areas with lower microalgal biomass. All surface sediment parameters varied rhythmically between day- and night-time. Significant differences were found between day- and night-time biostabilisation coefficients, however, differences between day- and night-time τ c values were not detected. It is suggested that sediment stability at night is enhanced in high microalgal biomass areas as a result of degradation products from bound extra-cellular polysaccharides (EPS) not easily detected using standard extraction procedures.

Paleoecology of late-glacial terrestrial deposits with in situ conifers from the submerged continental shelf of western canada

Extensive portions of the continental shelf off the coast of British Columbia were subaerially exposed during Late Wisconsinan deglaciation due to lowering of relative sea level by as much as 150 m. Paleoecological analyses were conducted at two sites on the emergent continental shelf where terrestrial surfaces with in situ conifers are preserved. The woody plant remains confirm that, during the latest period of subaerial exposure, terrestrial vegetation was established on the continental shelf. Microscopic identification of fossil wood, and analyses of pollen and plant macrofossils from the associated paleosols and overlying shallow pond sediments indicate that productive Pinus contorta-dominated communities with abundant Alnus crispa and ferns grew on the shelf adjacent to and on the Queen Charlotte Islands around 12,200 14C yr B.P. Dwarf shrubs including Salix and Empetrum, and herbaceous plants such as Heracleum lanatum and Hippuris vulgaris, were also important components of the shelf vegetation. Near northern Vancouver Island, mixed coniferous forests dominated by Pinus contorta with Picea, Tsuga spp., Alnus spp., and ferns occupied the shelf at 10,500 14C yr B.P.

Late Messinian to Early Pliocene paleoenvironmental changes in the Melilla Basin (NE Morocco) and their relation to Mediterranean evolution

Three major paleoenvironmental changes have been recognized during the late Miocene to Early Pliocene in the Melilla Basin (Northeastern Morocco) and compared with the regional events that affected the Mediterranean hydrology during this crucial period. The first change was the definitive conversion of the restricted marine conditions that prevailed since the end of the reef carbonate complex into lacustrine environments; this occurred around 5.8 Ma which is earlier than in the rest of the Mediterranean where the Lower Evaporites were still forming. These lacustrine settings display great similarities with the Lago-Mare environments that started in the Mediterranean during the deposition of the Upper Evaporites and climaxed during the latest Messinian. The second change corresponds to a long period of subaerial exposure that caused a widespread erosional surface; this resulted in a deep paleovalley, which truncates the Messinian sedimentary succession of the deeper southwestern part of the basin. This event postdates a lava flow dated at 5.77 Ma and confirms the importance of the latest Messinian erosional event. The third change was the infilling of this topography by marine sediments when the basin was rapidly reflooded in Early Pliocene. The lack of evaporites indicates that the Melilla Basin did not experience the Messinian Salinity Crisis. This could be explained either by the drainage of the brines towards the deeper South Alboran Basin or rather by an early dilution of the basin due to local excess of continental water inputs while most of the other Mediterranean basins evolved at the same time into evaporitic settings. These data confirm that the Rifian gateway was completely closed, thus, preventing Atlantic water inputs in the Mediterranean during the whole evaporitic deposition and the Lago-Mare episode. Such an evolution fits well with the history of depositional environments in the Alboran domain where evaporites are poorly developed and the Messinian is mainly represented by an erosional surface.

Subbottom acoustic and sedimentary records of past surface water?groundwater exchange through sinkhole lakes in south Georgia, U.S.A

Lake Balboa is a deep sinkhole lake in south Georgia. Subbottom acoustic profiling and long sediment cores reveal four stratigraphic units within >20 m of lacustrine fill above acoustically stratified and faulted Miocene/Pliocene overburden and basement Eocene limestone which hosts the Floridan Aquifer system (FAS). Fill consists of thin, rhythmically bedded, peaty sediments (Unit I), indicative of slow sedimentation in a shallow swamp until 13,110–12,680 cal BP, a mix of desiccated and over-consolidated clay and silty sand (Unit II) implying periodic subaerial exposure of the lake bed until 9470–9025 cal BP, and gyttja associated with a rapid 8 m rise in water level by 9120–9020 cal BP (Unit III), as well as an additional rise of 10 m to modern lake levels (Unit IV). Accumulation of nearly 4.5 m of low permeability gyttja, and higher lake levels have fundamentally changed exchange between surface water and groundwater, reducing by 40% or more the rate of vertical leakage to the FAS and increasing shallow aquifer discharge to the lake.

An Assessment of Stratigraphic Completeness in Climate-Sensitive Closed-Basin Lake Sediments: Salar de Atacama, Chile

ABSTRACT Dated subsurface cores, 40 m, 100 m, and 200 m in length, from the Salar de Atacama, Chile, record changes in climate and tectonics over the past 325 ka. These cores were used to assess completeness of the stratigraphic record and how that record was influenced by faulting and climate change. The same basic facies, efflorescent halite crust and cement (subaerial halite), and chevron halite (saline lake) occur in each core. The dominant facies is efflorescent halite formed in a subaerial environment like that in the Atacama basin today. Thick, well-preserved efflorescent crusts in the Salar de Atacama cores illustrate that major sediment aggradation occurred in subaerial environments by growth of efflorescent halite from evaporation of groundwater brines. Similar pedogenically formed evaporites may be pre served in the geologic record. The N-S trending Salar Fault System has downdropped the eastern block of the Salar de Atacama (core 2002) relative to the western block (cores 2005 and 2031). Relative faulting rates over the past 60 ka were determined from stratigraphic offset and uranium-series ages from core 2005. Major faulting along the Salar Fault System occurred between 16.5 and 5.4 ka, with offset of 26.5 m and faulting rates of 2.4 m/kyr. Aggradation of efflorescent halite crusts on the downdropped eastern block served to smooth out topographic variations created by faulting. Although stratigraphic units in core 2002 shifted down > 30 m over the past 60 ka, the thickness and sedimentary features are similar for all but one equivalent stratigraphic unit in all three cores. Faulting along the Salar Fault System, despite significant offset, did not alter the basic stratigraphic record. Temporal completeness of the stratigraphic record, within the limits of the age dates and their errors of greater than ± 3 ka, was examined qualitatively by comparing long-term sedimentation rates (0.6-0.9 m/kyr) and shorter-term sedimentation rates (0.3-3.6 m/kyr). The similarity of sedimentation rates on the 10 kyr scale and the longer 100 to 300 kyr scale suggests temporal completeness of Salar de Atacama sediments, at least at the 10 kyr scale. Although hiatuses, long periods of subaerial exposure, and dissolution of evaporites may have occurred, the main climate fluctuations at Salar de Atacama over the last 100 ka are recorded in all three subsurface cores. Paleoclimate records obtained from such cores are therefore representative of basin-wide climate change and are valid for regional paleoclimate reconstructions.

Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record

Interest in palustrine carbonates and calcretes has increased over the last 20 years since they contain significant environmental information. Much of the work performed in this area has focused on either of two types of terrestrial carbonate—palustrine carbonates or calcretes (pedogenic and groundwater)—yet their simultaneous study shows there may be a gradual transition from one form to the other, revealing the interplay between pedogenic, sedimentary, and diagenetic processes. Three main factors control the formation of these carbonates: the position of the water table, the host rock, and the period of sub-aerial exposure. In pedogenic calcretes, precipitation of carbonate takes places mostly in the vadose zone above the water table, and within a previous host rock or sediment. In groundwater calcretes, the precipitation of carbonate also occurs within a previous host rock and around the groundwater table. In palustrine carbonates, however, the precipitation of lime mud occurs in a lacustrine water body. Palustrine carbonates necessarily form on previous lacustrine mud, whereas both types of calcretes may form on any type of sediment or soil. The sub-aerial exposure time needed to form palustrine carbonates may by relatively short (even a season), whereas pedogenic calcretes need more time (several years to millions of years). Groundwater calcretes do not form on the topographic surfaces, so there is no need of sub-aerial exposure. However, stable surfaces favour the development of thick groundwater calcretes. Small fluctuations in the water table cause gradual transitions of these three types of terrestrial carbonates and the subsequent mixture of their characteristic features, causing difficulties in the interpretation of these carbonates. The formation of these carbonates is controlled by palaeoenvironmental factors. Both commonly form in semi-arid climates. Arid climates are also suitable for calcretes, but sub-humid conditions are more suitable for palustrine carbonates. More indications of climatic conditions may be obtained through the analysis of the d18O content of both calcretes and palustrine carbonates, and from the depth of the horizon containing carbonate nodules in pedogenic calcretes. Vegetation is also important in the formation of these types of carbonates. Data on the prevailing vegetation can be obtained from the analysis of the micro and macrofabric as well as from the d13C signal of the primary carbonates, which, in pedogenic carbonates, has also been used to estimate atmospheric pCO2 during the Phanerozoic. These terrestrial carbonates are widely distributed on floodplains and distal areas of alluvial basins. Their presence and characteristics can be used as indicators of aggradation, subsidence or accommodation rates, and therefore as indicators of different tectonic regimes. Even though the study of these carbonates has notably increased in recent years, much less is known about them than about marine carbonates. Presently, there is much emphasis on obtaining a general model for sequence stratigraphy in terrestrial basins, with a need to include the carbonates analysed in this paper.

High resolution sequence stratigraphic architecture of a transgressive coastal succession: Albian Bow Island Formation, southwestern Alberta

ABSTRACT We here present the first detailed published sequence stratigraphic study of the Lower Cretaceous Bow Island Formation. Two orders of sequences are identified, based on the integration of sedimentology, micropaleontology, ichnology and petrophysical characteristics. The studied interval encompasses two lower-order sequences. Shelf, shoreface, and coastal plain deposits of the lower and middle members of the Bow Island Formation form the transgressive and highstand systems tracts of the basal sequence. A major relative sea level fall caused a prolonged period of sub-aerial exposure and weathering of the deposits of the middle member of the Bow Island Formation, and farther to the east and northeast, incision of deep valleys into shoreface deposits of the Viking Formation. An overlying complex succession of tidal flat, estuarine, shoreface, and shelf deposits of the upper member of the Bow Island Formation and marine shales of the Westgate Formation represent the transgressive systems tract of the younger low-order sequence. Within the transgressive succession of the upper member of the Bow Island Formation and Westgate Formation, five higher-order sequences are identified. The high-order sequence boundaries are marked by basinward shifts of facies, incision of up to 15 m deep valleys and soil development on intervalley areas. This demonstrates that this period of overall sea level rise was punctuated by relative sea level falls, justifying subdivision into sequences rather than parasequences. Transgressive surfaces, often developed as prominent, pebble-mantled ravinement surfaces, subdivide the individual highorder sequences into lower and upper units. The lower units are comprised of tidal influenced incised valley fill, forming the main reservoirs, and tidal and storm influenced back-barrier lagoons deposited during lowstand and early transgressive periods. Foraminiferal assemblages were critical in distinguishing between these lagoonal deposits and open marine shoreface deposits. Deposits of the basal part of the upper units are characterized by a stepwise change towards more open marine conditions following each transgression, from marginal marine tidal flat and lagoonal deposits to shoreface and finally to shelf deposits of the Westgate Formation. This demonstrates the stacking in a retrogradational sequence set of the high-order sequences of the upper member of the Bow Island Formation and Westgate Formation. Erosion associated with the transgressions increases upward in intensity with each successive transgression with the greatest depth of erosion associated with the Westgate transgression. This reflects an upward increase in energy regime of the sedimentary environments that bypassed the area during the transgression. Within the overall transgressive upper member of the Bow Island Formation, an upward increase in the depth of incised valleys with each successive incision event likely reflects an upward increase in depositional relief due to the development of more open marine conditions and to the retrogradational and aggradational character of the upper member of the Bow Island Formation. The Westgate transgressive surface is a complex ravinement surface characterized by a westward, stepwise rise, indicating that it was formed during a period of gradual relative sea level rise, interrupted by phases of more rapid sea level rise. Establishment of this high-resolution sequence stratigraphic framework allows differentiation between these various generations of incision and thereby the mapping of the individual valleys and reservoirs associated with them. * Current address: Petroleum Geology, Saskatchewan Industry and Resources, 201 Dewdney Ave East, Regina, SK, Canada, S4N 4G3; E-mail: ppedersen@ir.gov.sk.ca End_Page 441------------------------

Karst-controlled diagenesis and reservoir development: Example from the Ordovician main-reservoir carbonate rocks on the eastern margin of the Ordos basin, China

The Ordovician Majiagou Formation contains the main reservoir of the Ordos Central gas field in the Ordos basin. The producing zone at the top of member 5 consists of carbonate rocks modified during a long period of subaerial exposure and karstification from the Late Ordovician to the middle Carboniferous. On the eastern margin of the Ordos basin, the porosity of the exposed carbonate rocks of this unit ranges from 0.5 to 15.1%, and permeability ranges from 8%, and permeability ranges from <0.1 to 5 md. The main reservoir porosity is a dissolution-enhanced vuggy porosity, associated with dolomite. The carbonate rocks show great heterogeneity, reflecting the varying effects of karstification in creating and modifying porosity. Petrographic and geochemical analyses of various components in these carbonates provided evidence for depositional and diagenetic processes. The reservoir carbonates were deposited in shallow and restricted hypersaline environments and were later modified by karstification and burial diagenesis. Dolomitization appears to have resulted from mixing of marine and meteoric waters and probably occurred in both shallow and deep burial settings. Cementation by calcite also occurred in both shallow and deep environments, under different hydrodynamic conditions. Both depositional settings and diagenetic processes, such as leaching by meteoric water, paleokarstification, dolomitization, and cementation, controlled reservoir development. The outcrop and subsurface samples show similar petrographic features, porosity types, and geochemical characteristics, but the exposed section of the formation shows evidence of more alteration by meteoric water.

Early dolomitisation of Late Triassic platform carbonates in the Transdanubian Range (Hungary)

Abstract In the Late Triassic, a large carbonate platform system came into existence in the Transdanubian Range (TR) producing a 1.5- to 2-km-thick carbonate succession. The lower part of the platform carbonate series is made up of cyclic peritidal dolomite (Main Dolomite Formation), while the upper one consists of cyclic peritidal limestone (Dachstein Limestone Formation). However, there is a transitional member of remarkable thickness between them consisting of partially dolomitised limestone and calcitic dolomite. This study focuses on the transitional member where the process of early dolomitisation was halted at various stages. In the investigated sections the early dolomitisation was controlled by the climate and high-frequency sea-level oscillation, which led to periodic subaerial exposure of large areas and palaeo-hydrological conditions. Increasing humidity and gradual extension of the permanently inundated outer platform led to cessation of early dolomitisation in the Middle to Late Norian.

Regional-scale hydrogeology of the Upper Devonian-Lower Cretaceous sedimentary succession, south-central Alberta basin, Canada

The flow of formation waters in the Upper Devonian-Lower Cretaceous sedimentary succession in the south-central part of the Alberta basin is controlled mainly by (1) outcrops of Devonian and Mississippian strata at high elevation in the south in Montana, and at low elevation at the Peace River in the north; (2) subcrops of Devonian to Jurassic strata at the sub-Cretaceous unconformity; and (3) deposition of Cretaceous sediments on pre-Cretaceous relief exposed for a long period of time. Weathering of Upper Devonian strata during this long period of subaerial exposure and the concurrent paleokarsting of the Grosmont Formation led to high permeability in these aquifers. As a result, the Grosmont aquifer and the Upper Devonian aquifer system that subcrop at the sub-Cretaceous unconformity form a drainage path in a northward regional-scale flow system in the southern and central parts of the Alberta basin. This long-range flow system is fed by meteoric recharge in the south, by updip flow of connate waters from deep Paleozoic aquifers that subcrop along the western flank of the main system, and by downdip meteoric recharge through Cretaceous strata along the eastern basin edge. A plume of relatively high salinity is formed in the Lower Mannville aquifer in the area where highly saline Devonian waters discharge at the sub-Cretaceous unconformity and mix with fresh water of meteoric origin. Hydrocarbons generated in Upper Devonian to Lower Cretaceous strata of the deep foreland basin migrated northeastward updip, driven by buoyancy and supported by a concurrent hydrodynamic drive. The great majority of the generated hydrocarbons reached the sub-Cretaceous unconformity, where they were trapped in complex stratigraphic traps in the Lower Mannville Formation. Downward flow of meteoric water along (Begin page 638) the eastern flank of the basin hydrodynamically enhanced the trapping and led to hydrocarbon biodegradation in place into heavy oils and oil sands in the Cold Lake and Athabasca areas.

Post-Miocene subtropical karst evolution, lower Suwannee River basin, Florida

Morphometric characteristics of ∼25 000 karstic depressions suggest that the last phase of the post-Miocene karstic evolution within the lower Suwannee River basin of Florida has been controlled by the lower sea-level stands of the Pleistocene and the formation of the Suwannee River. During the Pleistocene, as interglacial seas retreated, marine terraces formed by sequential sea-level lowstands and the time period of subaerial exposure diminished toward the sea. Consequently, geomorphologically younger karst landforms formed as the elevation of marine terraces decreased. The evolutionary geomorphological development of this heavily karstified region produced more frequent and/or larger and more complex depressions at higher elevations. A geographic information system analysis of morphometric and spatial distribution parameters of the karstic depressions within the lower Suwannee River basin reveals that the Florida karst is represented by broad, shallow depressions with an average density of 6.07/km 2 and an average pitting index of 14.5. Morphometric and spatial distribution parameters of karstic depressions show a great variation within the lower Suwannee River area and thus preclude a simple morphoclimatic classification of karst landforms. The Tertiary carbonate rocks of the subtropical Florida karst have relatively less joint frequency and significant primary porosity, and do not produce the extreme karst landforms observed in the massive limestones of the tropics.

Dedolomitization and other early diagenetic processes in Miocene lacustrine deposits, Ebro Basin (Spain)

Abstract A variety of meteoric diagenetic features reveal the development of a syngenetic karst on lacustrine deposits of the Ebro Basin. Diagenetic processes that operated on lacustrine laminated and stromatolitic carbonates include the following. (1) A first syndepositional stage with processes such as dolomitization, desiccation and related breccia formation and sulphate precipitation, either as lenticular gypsum crystals or nodules. This stage took place under progressive evaporation due to lake level fall, when the previous carbonate deposits became exposed as a supra-littoral fringe surrounding saline mud flats of adjacent sulphate depositional environments. (2) A second early diagenetic stage in which processes such as sulphate dissolution and collapse brecciation, dedolomitization, calcite spar cementation and silicification occurred as a result of meteoric water input that caused a progressive rise in lake level. Light isotopic compositions (δ 13 C and δ 18 O) of diagenetic calcites, versus heavier compositions in primary laminated and stromatolitic limestones, confirm a meteoric influence. The syngenetic karst is best developed at the boundary between two allostratigraphic units and coincided with one of the extensive stages of sulphate deposition at the end of the Early Miocene. The karst facies occurred in an area that was a low-relief barrier that separated two sites of sulphate deposition during low lake levels. This indicates that the karst development was controlled by topographic changes within the basin and record a shift from arid to wetter climatic conditions, as suggested by the overlying freshwater carbonate deposits. The presence of diagenetic features such as those described in the central Ebro Basin affecting saline lacustrine carbonates is relevant because they can be used as indicators of subaerial exposure periods in terrestrial environments and they also reveal important palaeogeographic and palaeoclimatic events of basinal extent.

Erosion and sedimentation associated with the last sea level rise offshore Hong Kong, South China Sea

Abstract High-resolution seismic data reveals an extensive planar, seaward-dipping reflector offshore south of Hong Kong. This is interpreted as a ravinement surface created during the last transgression and is associated with ca. 2.5 m of shoreface sand, which is overlain by a 6–15 m thick sheet of prodelta-marine shelf sediments. On seismic profiles, the shoreface sand is characterised by a chaotic reflector pattern and the sheet by low-amplitude, parallel, flat or low-angle, continuous reflectors. Particle size data reveal a westward-coarsening and decrease in sorting, which reflects increasing proximity to the Pearl River. Foraminiferal and diatom assemblages demonstrate the same overall westward-shallowing trend, with increase in inner shelf foraminifera and brackish planktonic diatom species. A phase of early Holocene shoreface sand and intertidal silt deposition in eastern and central areas culminated in a brief period of subaerial exposure. This was succeeded by an extensive period of shelf and inner shelf deposition, brought about by variation in balance between the rate of sea-level rise and accommodation, and the discharge and sediment supply from the Pearl River.

Pseudospherulitic fibrous calcite in paleo-groundwater, unconformity-related diagenetic carbonates (Paleocene of the Ager Basin and Miocene of the Madrid Basin, Spain)

ABSTRACT Massive, decimeter-thick intervals formed by mosaics of subspherical calcite crystals displaying an intracrystalline fibrous microfabric have been found in Paleocene and Miocene strata in Spain. The fibrous microfabric is marked by inclusion-defined fibers that radiate from the central parts of the crystals, which can be termed pseudospherulites and not spherulites, because they are single crystals and not polycrystalline aggregates. The horizons of pseudospherulitic calcite appear in meter-scale, laterally persistent levels of crystalline limestones that exhibit complex pseudospar fabrics, they are interbedded with peritidal and/or lacustrine deposits, they grade downward into bedded dolomicrites, and they can be linked to unconformities several meters higher in the section, above intervening unaltered limestones. The pseudospar zones are interpreted as early-diagenetic, paleo-groundwater alteration products after dolomicrites. The pseudospherulites are composed of low-Mg calcite with low contents of Fe and Mn, and they have a stable-isotope composition indicative of precipitation from meteoric water. They form xenotopic mosaics in which goethite, palygorskite, and sepiolite may be present as inclusions and in the intercrystalline matrix. Cathodoluminescence microscopy reveals peripheral growth of the pseudospherulites, which results from several phases of precipitation and even of dissolution. The first phase of crystal growth may show rhombic habits and may contain corroded dolomite inclusions, interpreted as an indication of growth at the expense of dolomicrite by a dissolution/precipitation process. The second phase of crystal growth is fibrous sensu stricto, exhibits lobate growth laminae, and shows indications of displacive growth. The pseudospherulitic fabric is not relict and the pseudospherulites do not represent speleothems, recrystallized Microcodium, or neomorphosed spherulitic carbonate. Instead, they are interpreted as split crystals that grew within dolomicritic host sediments, in part by dissolution-precipitation and in part by displacement. The hypothesis of a dolomicritic host is supported by the geological and diagenetic setting, by the presence of corroded dolomite inclusions within the pseudospherulites, and by the relationship between their growth and the authigenesis of sepiolite, palygorskite, and goethite. The pseudospherulites grew in the shallow subsurface, at or near the water table, from predominantly oxidizing meteoric water, and under periodically fluctuating geochemical conditions (saturation, redox) caused by fluctuations of the water table during periods of subaerial exposure. The presence of fossil bacteria within the pseudospherulites, the anastomosing pattern of the inclusion-defined fibers, and the lobate growth forms of the fibrous calcite suggest some bacterial influence in the genesis of the pseudospherulitic fabric.

Preservation of pre-vegetational mixed fluvio–aeolian deposits in a humid climatic setting: an example from the Middle Proterozoic Eriksfjord Formation, Southwest Greenland

Sedimentological studies of a 30 m thick coastal cliff section within the Middle Proterozoic Eriksfjord Formation in western South Greenland reveals three distinct types of fluvial sand sheet deposits that reflect perennial streams (Type I), semi-perennial streams (Type II), and ephemeral flash floods (Type III). Perennial river sand sheets are characterised by co-sets of medium-scale trough cross-beds, interbedded with isolated medium- and large-scale, high-angle, tabular cross-beds. Indications of desiccation or subaerial exposure are absent. Semi-perennial fluvial sand sheets consist predominantly of low-angle cross-beds, interbedded with isolated sets of high-angle tabular cross-beds with common reactivation surfaces. Horizontal lamination and climbing ripple lamination form subordinate structures. Associated with the sand sheets are adhesion structures and 0.05–0.4 m thick sets of wind ripple-lamination indicating periods of subaerial exposure and aeolian reworking. High-energy ephemeral flash flood sand sheets consist almost exclusively of planar-parallel lamination and climbing ripple lamination with some isolated sets of low-angle cross-bedding. Scouring and internal truncation surfaces are common. The three types of sand sheets are considered to reflect deposition under changing climatic conditions, varying from humid to arid or semi-arid. Aeolian deposits are preserved within the sand sheets showing characteristics of dominantly perennial flow punctuated by shorter periods of desiccation (Type II), while sand sheets showing features typical of arid and or semi-arid flow conditions (Type III) contain no preserved aeolian deposits. This selective preservation is interpreted to be a result of the combined effect of groundwater table level and fluvial style which in turn are inferred to have been controlled by the climatic regime. The deposits show that during pre-vegetational times the preservation of aeolian deposits, under certain conditions, may be more optimal in fluvial systems formed in a humid climate than in fluvial systems formed under semi-arid or arid circumstances. The occurrence of aeolian deposits within a Precambrian succession of fluvial deposits therefore, need not be an indication of the most arid environmental conditions.

98/00903 The chemical structure and properties of bitumens of different origin: Baginska, K. and Gawel, I. Pet. Coal, 1996, 38, (2), 35–37

Petrographic analyses were carried out on thin coals and coaly sediments from the Lower Carboniferous Mattson Formation at Clausen Creek and Jackfish Gap-Yohin Ridge in the northern part of the Liard Basin, northern Canada. The composition and optical characteristics indicate that the coals are high-volatile bituminous B, predominantly sapropelic (canneloid) and accumulated subaquatically.The coals are dominantly composed of inertinite-rich and exinite-rich durities with subsidiary inertites and clarodurites; vitrite is minor and liptite is rare. The inertinite-rich microlithotypes are dominated by semifusinite, but micrinite, semimacrinite and ?resino-inertinites are abundant. Sporinite, comprising megaspores, crassispores, tenuispores and miospores, is the dominant liptinite maceral with subsidiary cutinite and minor alginite. Except for pyrite, mineral matter is minimal.Three populations of telocollinite are observed: a low-reflectance variety (I), commonly associated with micrinite (as vitrinertite), displays weak brown fluorescence and a reflectance some 0.4-0.5% lower than type II; type II is non-fluorescing telocollinite, with intermediate reflectance (0.67-0.74% Rom), it occurs as vitrite and is also associated with micrinite; and a higher-reflectance telocollinite (III), having no fluorescence or association with micrinite, has variable reflectance (0.74-0.8% Rom) implying higher oxidation or gelification levels.The abundance of semimacrinite, macrinite and ?resino-inertinites in inertites and durites (I) suggests that much of the peat accumulated subaquatically. Furthermore, fluorescing vitrinite and an abundance of micrinite (derived by oxidation or coalification of bituminite), suggest that the coal accumulated under anaerobic conditions. The predominance of semifusinite in humic laminae and micrinite in sapropelic layers suggests extensive surface or near-surface oxidation of the peat. Oxidised sporinites suggest that they were wind-borne.Depositional environment is interpreted as marginal marine, perhaps in shallow lakes in the middle to upper delta plain. Peat accumulations probably began subaquatically at the oxygen-hydrogen sulphide interface, but periodic subaerial exposure and natural oxidation gave rise to the high inertinite coals. Upper Mattson coals are interbedded with algal laminites and probably accumulated in a lagoonal setting.

Decreases in the swelling and porosity of bituminous coals during devolatilization at high heating rates

Petrographic analyses were carried out on thin coals and coaly sediments from the Lower Carboniferous Mattson Formation at Clausen Creek and Jackfish Gap-Yohin Ridge in the northern part of the Liard Basin, northern Canada. The composition and optical characteristics indicate that the coals are high-volatile bituminous B, predominantly sapropelic (canneloid) and accumulated subaquatically.The coals are dominantly composed of inertinite-rich and exinite-rich durities with subsidiary inertites and clarodurites; vitrite is minor and liptite is rare. The inertinite-rich microlithotypes are dominated by semifusinite, but micrinite, semimacrinite and ?resino-inertinites are abundant. Sporinite, comprising megaspores, crassispores, tenuispores and miospores, is the dominant liptinite maceral with subsidiary cutinite and minor alginite. Except for pyrite, mineral matter is minimal.Three populations of telocollinite are observed: a low-reflectance variety (I), commonly associated with micrinite (as vitrinertite), displays weak brown fluorescence and a reflectance some 0.4-0.5% lower than type II; type II is non-fluorescing telocollinite, with intermediate reflectance (0.67-0.74% Rom), it occurs as vitrite and is also associated with micrinite; and a higher-reflectance telocollinite (III), having no fluorescence or association with micrinite, has variable reflectance (0.74-0.8% Rom) implying higher oxidation or gelification levels.The abundance of semimacrinite, macrinite and ?resino-inertinites in inertites and durites (I) suggests that much of the peat accumulated subaquatically. Furthermore, fluorescing vitrinite and an abundance of micrinite (derived by oxidation or coalification of bituminite), suggest that the coal accumulated under anaerobic conditions. The predominance of semifusinite in humic laminae and micrinite in sapropelic layers suggests extensive surface or near-surface oxidation of the peat. Oxidised sporinites suggest that they were wind-borne.Depositional environment is interpreted as marginal marine, perhaps in shallow lakes in the middle to upper delta plain. Peat accumulations probably began subaquatically at the oxygen-hydrogen sulphide interface, but periodic subaerial exposure and natural oxidation gave rise to the high inertinite coals. Upper Mattson coals are interbedded with algal laminites and probably accumulated in a lagoonal setting.

POROSITY GENERATION AND RESERVOIR POTENTIAL OF OULDBURRA FORMATION CARBONATES, OFFICER BASIN, SOUTH AUSTRALIA

Porosity generation and reservoir potential of the early Cambrian Ouldburra Formation in the eastern Officer Basin is delineated by combining petrographical, petrophysical and sedimentological studies. The shallow marine Ouldburra Formation consists of carbonates, mixed carbonates and clastics, clastics and evaporites. Detailed analysis of more than 100 samples shows that dolomitisation resulted in substantial secondary porosity development within the carbonates. Secondary porosity has also been generated within the mixed siliciclastic-carbonate zone by carbonate matrix and grain dissolution as well as by dolomitisation. Prospective reservoir units correspond to highstand shallow marine facies where short periods of subaerial exposure resulted in diagenetic changes.Sedimentary facies and rock character indicate that sabkha and brine reflux models are applied to dolomitisation within the Ouldburra Formation. Dolomite mainly occurs in two stages: common anhedral dolomites formed early by replacement of pre-existing limestone, and saddle dolomite and coarse crystalline dolomite formed during the late stages of burial diagenesis, associated with hydrocarbon shows. The dolomite reservoirs identified are ranked on the basis of their porosity distribution and texture into groups I to IV. Dolomites with rank I and II exhibit excellent to good reservoir characteristics respectively.The Ouldburra Formation shows many depositional and diagenetic similarities to the Richfield Member of the Lucas Formation in the Michigan Basin of the USA. Substantial oil and gas production from middle Devonian shallow water to sabkha dolomites makes the Richfield Member an attractive reservoir analogue to the Ouldburra Formation.

Facies and diagenesis of some upper proterozoic dolomites of South China

The Dengying Formation exposed at Sansha, Human Province, central South China, comprises unfossiliferous dolomitized platform carbonates and a solution-collapse breccia sequence. Microfacies analysis indicates a cyclic, increasingly restricted lagoonal to oolithic platform marginal facies. Sedimentation was repeatedly interrupted by periods of subaerial exposure as suggested by features of meteoric diagenesis and karstification. During a final regressive phase selective karst-dissolution of strata, interpreted as dolomite-evaporite interbeds, created a solution-residual/collapse breccia sequence. The Dengying Fm. is disconformably overlain by Lower Cambrian transgressive phosphatic conglomerates and fossiliferous black shales. These local facies data add another piece to the puzzle of Proterozoic/Cambrian Yangtze platform sedimentational history and demonstrate the applicability of microfacies analysis to uppermost Proterozoic strata.