Geology Reports⌕ Search

Geology topics

R. M. Flores

Publications and source records attributed to R. M. Flores.

At least 19 recordsLinked to original sources
USGS2008

Methanogenic pathways of coal-bed gas in the Powder River Basin, United States: The geologic factor

Coal-bed gas of the Tertiary Fort Union and Wasatch Formations in the Powder River Basin in Wyoming and Montana, U.S. was interpreted as microbial in origin by previous studies based on limited data on the gas and water composition and isotopes associated with the coal beds. To fully evaluate the microbial origin of the gas and mechanisms of methane generation, additional data for 165 gas and water samples from 7 different coal-bed methane-bearing coal-bed reservoirs were collected basinwide and correlated to the coal geology and stratigraphy. The C1/(C2 + C3) ratio and vitrinite reflectance of coal and organic shale permitted differentiation between microbial gas and transitional thermogenic gas in the central part of the basin. Analyses of methane ??13C and ??D, carbon dioxide ??13C, and water ??D values indicate gas was generated primarily from microbial CO2 reduction, but with significant gas generated by microbial methyl-type fermentation (aceticlastic) in some areas of the basin. Microbial CO2 reduction occurs basinwide, but is generally dominant in Paleocene Fort Union Formation coals in the central part of the basin, whereas microbial methyl-type fermentation is common along the northwest and east margins. Isotopically light methane ??13C is distributed along the basin margins where ??D is also depleted, indicating that both CO2-reduction and methyl-type fermentation pathways played major roles in gas generation, but gas from the latter pathway overprinted gas from the former pathway. More specifically, along the northwest basin margin gas generation by methyl-type fermentation may have been stimulated by late-stage infiltration of groundwater recharge from clinker areas, which flowed through highly fractured and faulted coal aquifers. Also, groundwater recharge controlled a change in gas composition in the shallow Eocene Wasatch Formation with the increase of nitrogen and decrease of methane composition of the coal-bed gas. Other geologic factors, such as burial, thermal and maturation history, lateral and vertical continuity, and coalification of the coal beds, also played a significant role in controlling methanogenic pathways and provided new perspectives on gas evolution and emplacement. The early-stage gas produced by CO2 reduction has mixed with transitional thermogenic gas in the deeper, central parts of the Powder River Basin to form 'old' gas, whereas along the basin margins the overprint of gas from methyl-type fermentation represents 'new' gas. Thus, a clear understanding of these geologic factors is necessary to relate the microbiological, biogeochemical, and hydrological processes involved in the generation of coal-bed gas.

International Journal of Coal Geology↗
USGS2006

A new reference section for palynostratigraphic zonation of Paleocene rocks in the Rocky Mountain region

A biostratigraphic (palynostratigraphic) zonation of Paleocene rocks was established in the northeastern Wind River Basin near Waltman, Natrona County, Wyoming, in 1978 and subsequently applied extensively by various workers throughout the Rocky Mountain region. Because the original study on which the zonation was based was proprietary, precise details about the locations of the two reference sections and the samples on which the zonation was based were not published and are no longer retrievable. Therefore, it is useful (although not required) to designate formally a new reference section for the Paleocene biozones. Accordingly, exposures of Paleocene and associated strata within and west of the Castle Gardens Petroglyph Site in Fremont County, Wyoming, in the east-central part of the Wind River Basin, were selected for this purpose. At this location, composite stratigraphic sections encompassing 740 m of strata were measured, described, and sampled. Productive samples yielded characteristic Maastrichtian palynomorphs from the lower part of the sampled interval and diagnostic species of the six palynological biozones zones widely known as P1 (lower Paleocene) through P6 (upper Paleocene), through an interval of about 580 m. The Paleocene biozones are present in the same consistent stratigraphic order in the Castle Gardens area as observed in the 1978 study and subsequent studies throughout the Rocky Mountain region. In accordance with the North American Stratigraphic Code, the historical background is presented; intent to establish the Castle Gardens reference section is declared; the category, rank, and formal names of biostratigraphic units within it are specified; and the features of the biozonation are described, including biozone boundaries, ages, and regional relations. Occurrences of biostratigraphically significant palynological species within each biozone in the reference section are tabulated, and presence of these and other species in correlative biozones are discussed. The new reference section in the Castle Gardens area replaces the original reference sections near Waltman and provides a better-documented foundation for Paleocene palynostratigraphy in the Rocky Mountain region.

Mountain Geologist↗
USGS2001

Coal-bed gas resources of the Rocky Mountain region

The Rocky Mountain region contains several sedimentary provinces with extensive coal deposits and significant accumulations of coal-bed gas. This summary includes coal-bed gas resources in the Powder River Basin (Wyoming and Montana), Wind River Basin (Wyoming), Southwest Wyoming (Greater Green River Basin of Wyoming, Colorado, and Utah), Uinta-Piceance Basin (Colorado and Utah), Raton Basin (Colorado and New Mexico), and San Juan Basin (Colorado and New Mexico). Other provinces in the Rocky Mountain region may contain significant coal-bed gas resources, but these resource estimates are not available at this time.

Colorado, Montana, New Mexico, Utah, Wyoming↗
USGS1998

Developmental geology of coalbed methane from shallow to deep in Rocky Mountain basins and in Cook Inlet-Matanuska Basin, Alaska, USA and Canada

The Rocky Mountain basins of western North America contain vast deposits of coal of Cretaceous through early Tertiary age. Coalbed methane is produced in Rocky Mountain basins at depths ranging from 45 m (150 ft) to 1981 m (6500 ft) from coal of lignite to low-volatile bituminous rank. Although some production has been established in almost all Rocky Mountain basins, commercial production occurs in only a few. despite more than two decades of exploration for coalbed methane in the Rocky Mountain region, it is still difficult to predict production characteristics of coalbed methane wells prior to drilling. Commonly cited problems include low permeabilities, high water production, and coals that are significantly undersaturated with respect to methane. Sources of coalbed gases can be early biogenic, formed during the early stages of coalification, thermogenic, formed during the main stages of coalification, or late stage biogenic, formed as a result of the reintroduction of methane-gnerating bacteria by groundwater after uplift and erosion. Examples of all three types of coalbed gases, and combinations of more than one type, can be found in the Rocky Mountain region. Coals in the Rocky Mountain region achieved their present ranks largely as a result of burial beneath sediments that accumulated during the Laramide orogeny (Late Cretaceous through the end of the eocene) or shortly after. Thermal events since the end of the orogeny have also locally elevated coal ranks. Coal beds in the upper part of high-volatile A bituminous rank or greater commonly occur within much more extensive basin-centered gas deposits which cover large areas of the deeper parts of most Rocky Mountain basins. Within these basin-centered deposits all lithologies, including coals, sandstones, and shales, are gas saturated, and very little water is produced. The interbedded coals and carbonaceous shales are probably the source of much of this gas. Basin-centered gas deposits become overpressured from hydrocarbon generation as they form, and this overpressuring is probably responsible for driving out most of the water. Sandstone permeabilities are low, in part because of diagenesis caused by highly reactive water given off during the early stages of coalification. Coals within these basin-centered deposits commonly have high gas contents and produce little water, but they generally occur at depths greater than 5000 ft and have low permeabilities. Significant uplift and removal of overburden has occurred throughout the Rocky Mountain region since the end of the Eocene, and much of this erosion occurred after regional uplift began about 10 Ma. The removal of overburden generally causes methane saturation levels in coals to decrease, and thus a significant drop in pressure is required to initiate methane production. The most successful coalbed methane production in the Rocky Mountain region occurs in areas where gas contents were increased by post-Eocene thermal events and/or the generation of late-stage biogenic gas. Methane-generating bacteria were apparently reintroduced into the coals in some areas after uplift and erosion, and subsequent changes in pressure and temperature, allowed surface waters to rewater the coals. Groundwater may also help open up cleat systems making coals more permeable to methane. If water production is excessive, however, the economics of producing methane are impacted by the cost of water disposal.The Rocky Mountain basins of western North America contain vast deposits of coal of Cretaceous through early Tertiary age. Coalbed methane is produced in Rocky Mountain basins at depths ranging from 45 to 1981 m from coal of lignite to low volatile bituminous rank. Despite more than two decades of exploration for coalbed methane in Rocky Mountain region, it is still difficult to predict production characteristics of coalbed methane wells prior to drilling. Sources of coalbed gases can be early biogenic, formed during the main stages of coa

International Journal of Coal Geology↗
USGS1998

Coalbed methane: from hazard to resource

Coalbed gas, which mainly consists of methane, has remained a major hazard affecting safety and productivity in underground coal mines for more than 100 yr. Coalbed gas emissions have resulted in outbursts and explosions where ignited by open lights, smoking or improper use of black blasting powder, and machinery operations. Investigations of coal gas outbursts and explosions during the past century were aimed at predicting and preventing this mine hazard. During this time, gas emissions were diluted with ventilation by airways (eg, tunnels, vertical and horizontal drillholes, shsfts) and by drainage boreholes. The 1970s 'energy crisis' led to studies of the feasibility of producing the gas for commercial use. Subsequent research on the origin, accumulation, distribution, availability, and recoverability has been pursued vigorously during the past two decades. Since the 1970s research investigations on the causes and effects of coal mine outbursts and gas emissions have led to major advances towards the recovery and development of coalbed methane for commercial use. Thus, coalbed methane as a mining hazard was harnessed as a conventional gas resource.Coalbed gas, which mainly consists of methane, has remained a major hazard affecting safety and productivity in underground coal mines for more than 100 years. Coalbed gas emissions have resulted in outbursts and explosions where ignited by open lights, smoking or improper use of black blasting powder, and machinery operations. Investigations of coal gas outbursts and explosions during the past century were aimed at predicting and preventing this mine hazard. During this time, gas emissions were diluted with ventilation by airways (e.g., tunnels, vertical and horizontal drillholes, shafts) and by drainage boreholes. The 1970's `energy crisis' led to studies of the feasibility of producing the gas for commercial use. Subsequent research on the origin, accumulation, distribution, availability, and recoverability has been pursued vigorously during the past two decades. Since the 1970's research investigations on the causes and effects of coal mine outbursts and gas emissions have led to major advances towards the recovery and development of coalbed methane for commercial use. Thus, coalbed methane as a mining hazard was harnessed as a conventional gas resource.

International Journal of Coal Geology↗
USGS1996

Modern Pearl River Delta and Permian Huainan coalfield, China: A comparative sedimentary facies study

Sedimentary facies types of the Pleistocene deposits of the Modern Pearl River Delta in Guangdong Province, China and Permian Member D deposits in Huainan coalfield in Anhui Province are exemplified by depositional facies of anastomosing fluvial systems. In both study areas, sand/sandstone and mud/mudstone-dominated facies types formed in diverging and converging, coeval fluvial channels laterally juxtaposed with floodplains containing ponds, lakes, and topogenous mires. The mires accumulated thin to thick peat/coal deposits that vary in vertical and lateral distribution between the two study areas. This difference is probably due to attendant sedimentary processes that affected the floodplain environments. The ancestral floodplains of the Modern Pearl River Delta were reworked by combined fluvial and tidal and estuarine processes. In contrast, the floodplains of the Permian Member D were mainly influenced by freshwater fluvial processes. In addition, the thick, laterally extensive coal zones of the Permian Member D may have formed in topogenous mires that developed on abandoned courses of anastomosing fluvial systems. This is typified by Seam 13-1, which is a blanket-like body that thickens to as much as 8 in but also splits into thinner beds. This seam overlies deposits of diverging and converging, coeval fluvial channels of the Sandstone D, and associated overbank-floodplain deposits. The limited areal extent of lenticular Pleistocene peat deposits of the Modern Pearl River Delta is due to their primary accumulation in topogenous mires in the central floodplains that were restricted by contemporaneous anastomosing channels.

Conference Paper↗
USGS1996

Occurrence and morphology of carbonate concretions in the Beulah-Zap coal bed, Williston basin, North Dakota

Carbonate concretionary bodies were encountered during mining of the Beulah-Zap lignite seam in the Coteau Properties' Freedom mine, Mercer County, North Dakota. Preliminary studies show that areal and vertical distribution of the concretions are variable. All concretions examined are composed almost entirely of calcite. They occur as thin tabular bodies, as more or less elliptical forms, or as tear shaped bodies, and may occur individually or as clusters of buff-colored, poorly consolidated to solidly crystalline material. The carbonate masses vary in size from a few millimeters to tens of centimeters. Bedding in the lignite may display some compactional folding over dense spheroidal to elliptical concretions, indicating formation of the concretions prior to compaction. Internal morphology of the concretions is complex, and includes cone-in-cone structure, cross-cutting calcite veinlets, and multiple generations of calcite. Carbon isotope values suggest the concretions are composed of biogenic carbonate, probably related to early diagenesis and decomposition of organic matter (peat); oxygen isotope values are light, and consistent with a freshwater origin.

Conference Paper↗
USGS1996

Depositional controls on coal distribution and quality in the Eocene Brunner Coal Measures, Buller Coalfield, South Island, New Zealand

The Buller Coalfield on the West Coast of the South Island, New Zealand, contains the Eocene Brunner Coal Measures. The coal measures unconformably overlie Paleozoic-Cretaceous basement rocks and are conformably overlain by, and laterally interfinger with, the Eocene marine Kaiata Formation. This study examines the lithofacies frameworks of the coal measures in order to interpret their depositional environments. The lower part of the coal measures is dominated by conglomeratic lithofacies that rest on a basal erosional surface and thicken in paleovalleys incised into an undulating peneplain surface. These lithofacies are overlain by sandstone, mudstone and organic-rich lithofacies of the upper part of the coal measures. The main coal seam of the organic-rich lithofacies is thick (10-20 m), extensive, locally split, and locally absent. This seam and associated coal seams in the Buller Coalfield are of low- to high-volatile bituminous rank (vitrinite reflectance between 0.65% and 1.75%). The main seam contains a variable percentage of ash and sulphur. These values are related to the thickening and areal distribution of the seam, which in turn, were controlled by the nature of clastic deposition and peat-forming mire systems, marine transgression and local tidal incursion. The conglomeratic lithofacies represent deposits of trunk and tributary braided streams that rapidly aggraded incised paleovalleys during sea-level stillstands. The main seam represents a deposit of raised mires that initially developed as topogenous mires on abandoned margins of inactive braidbelts. Peat accumulated in mires as a response to a rise in the water table, probably initially due to gradual sea-level rise and climate, and the resulting raised topography served as protection from floods. The upper part of the coal measures consists of sandstone lithofacies of flu vial origin and bioturbated sandstone, mudstone and organic-rich lithofacies, which represent deposits of paralic (deltaic, barrier shoreface, tidal and mire) and marine environments. The fluvial sandstone lithofacies accumulated in channels during a sea-level stillstand. The channels were infilled by coeval braided and meandering streams prior to transgression. Continued transgression, ranging from tidal channel-estuarine incursions to widespread but uneven paleoshoreline encroachment, accompanied by moderate basin subsidence, is marked by a stacked, back-stepping geometry of bioturbated sandstone and marine mudstone lithofacies. Final retrogradation (sea-level highstand) is marked by backfilling of estuaries and by rapid landward deposition of the marine Kaiata Formation in the late Eocene.

International Journal of Coal Geology↗
USGS1996

Assessing the coal resources of the United States

In 1994, coal production in the United States reached the highest level in history (slightly more than 909 million metric tons or one billion short tons), continuing the upward trend of coal production and utilization that began 34 years ago. Previous assessments of the coal resources of the United States, which were completed as early as 1909, clearly indicated that the total coal resources of the Nation are large and that utilization at the current rate will not soon deplete them.

Fact Sheet↗
USGS1995

Overview of the influence of syn-sedimentary tectonics and palaeo-fluvial systems on coal seam and sand body characteristics in the Westphalian C strata, Campine Basin, Belgium

The Westphalian C strata found in the northeastern part of the former Belgian coal district (Campine Basin), which is part of an extensive northwest European paralic coal basin, are considered. The thickness and lateral continuity of the Westphalian C coal seams vary considerably stratigraphically and areally. Sedimentological facies analysis of borehole cores indicates that the deposition of Westphalian C coal-bearing strata was controlled by fluvial depositional systems whose architectures were ruled by local subsidence rates. The local subsidence rates may be related to major faults, which were intermittently reactivated during deposition. Lateral changes in coal seam groups are also reflected by marked variations of their seismic signatures. Westphalian C fluvial depositional systems include moderate to low sinuosity braided and anastomosed river systems. Stable tectonic conditions on upthrown, fault-bounded platforms favoured deposition by braided rivers and the associated development of relatively thick, laterally continuous coal seams in raised mires. In contrast, rapidly subsiding downthrown fault blocks favoured aggradation, probably by anastomosed rivers and the development of relatively thin, highly discontinuous coal seams in topogenous mires.

Campine Basin↗
USGS1994

A debris flow deposit in alluvial, coal-bearing facies, Bighorn Basin, Wyoming, USA: Evidence for catastrophic termination of a mire

Coal and clastic facies investigations of a Paleocene coal-bearing succession in the Grass Creek coal mine, southwestern Bighorn Basin, Wyoming, USA, suggest that disruption of peat accumulation in recurrent mires was caused by the repetitive progradation of crevasse splays and, ultimately, by a catastrophic mass movement. The mass movement, represented by deposits of debris flow, marked the termination of significant peat accumulation in the Grass Creek coal mine area. Megascopic and microscopic analyses of coal beds exposed along the mine highwalls suggest that these deposits developed in low-lying mires, as evidenced primarily by their ash yields and maceral composition. Disruption of peat accumulation in successive mires was caused by incursions of sediment into the mire environments. Termination by crevasse splay progradation is represented by coarsening-upward successions of mudrock and tabular, rooted sandstone, which overlie coal beds in the lower part of the coal-bearing interval. A more rapid process of mire termination by mass movement is exemplified by a debris flow deposit of diamictite, which overlies the uppermost coal bed at the top of the coal-bearing interval. The diamictite consists of a poorly sorted, unstratified mixture of quartzite cobbles and pebbles embedded in a claystone-rich or sandy mudstone matrix. Deposition of the diamictite may have taken place over a matter of weeks, days, or perhaps even hours, by catastrophic flood, thus reflecting an instantaneous process of mire termination. Coarse clastics and mud were transported from the southwest some 20–40 km as a viscous debris flow along stream courses from the ancestral Washakie Range to the Grass Creek area, where the flow overrode a low-lying mire and effectively terminated peat accumulation.

International Journal of Coal Geology↗
USGS1993

Geologic and geomorphic controls of coal development in some Tertiary Rocky Mountain basins, USA

Previous investigations have not well defined the controls on the development of minable coals in fluvial environments. This study was undertaken to provide a clearer understanding of these controls, particularly in of the lower Tertiary coal-bearing deposits of the Raton and Powder River basins in the Rocky Mountain region of the United States. In this region, large amounts of coals accumulated in swamps formed in the flow-through fluvial systems that infilled these intermontane basins. Extrabasinal and intrabasinal tectonism partly controlled the stratigraphic and facies distributions of minable coal deposits. The regional accumulation of coals was favored by the rapid basin subsidence coupled with minimal uplift of the source area. During these events, coals developed in swamps associated with anastomosed and meandering fluvial systems and alluvial fans. The extensive and high rate of sediment input from these fluvial systems promoted the formation of ombrotrophic, raised swamps, which produced low ash and anomalously thick coals. The petrology and palynology of these coals, and the paleobotany of the associated sediments, suggest that ombrotrophic, raised swamps were common in the Powder River Basin, where the climate during the early Tertiary was paratropical. The paleoecology of these swamps is identical to that of the modern ombrotrophic, raised swamps of the Baram and Mahakam Rivers of Borneo.

International Journal of Coal Geology↗
USGS1993

Reser­voir anisotropy and facies stratigraphic framework in the Pale­ocene Fort Union Formation, western Wind River basin

Investigation of reservoir anisotropy and lithofacies stratigraphic framework in the Fort Union Formation in western Wind River Basin, Wyoming focused on excellent surface exposures in the Shotgun Butte, Eagle Point, and Shotgun Bench synclines, and in the Merriam anticline area of the Wind River Reservation ( Fig. 1 ). A complementary study was made of the formation in the Muddy Ridge and Pavillion gas fields, 8-10 mi to the southeast ( Fig. 2 ). The Fort Union Formation is as much as 4000 ft thick in these areas, but thins to approximately 1800 ft toward the northern flank of the Little Dome anticline 3 mi south of Merriam anticline ( Keefer and Troyer, 1964 ). The Fort Union Formation includes interbedded conglomerates, sandstones, siltstones, mudstones, coals, and carbonaceous shales ( Fig. 3 ). The lower member of the Fort Union Formation is dominated by conglomerates and sandstones. The overlying Shotgun Member of the Fort Union Formation mainly consists of siltstones, mudstones, and carbonaceous shales, and coals, and subordinate sandstones. Contact between the lower member and Shotgun Member is gradational and marked by a topographic change from the resistant conglomerates and sandstones of the lower member to less resistant fine-grained strata of the Shotgun Member. In addition, the Shotgun Member commonly contains coal and carbonaceous shale beds, both in the surface and subsurface ( Fig. 4 ). About 15-20 mi east of the study area the Waltman Shale Member of the Fort Union Formation pinches out at the contact between the lower member and Shotgun Member (Keefer and Johnson, this volume). The Waltman Shale Member, which consists of brown to gray silty and shaly claystones interbedded with sandstones, increases in thickness to as much as 3000 ft eastward into the basin center (Keefer, 1961 ; 1965 ). Thus, eastward, the Paleocene Fort Union Formation in ascending order, contains the lower member, Waltman Shale Member, and Shotgun Member. The Shotgun Member generally thins and interfingers with the Waltman Member.

Conference Paper↗
USGS1993

Palynostratigraphic correlation of the Fort Union formation (Paleocene) in the Wind River Reservation and Waltman Area, Wind River Basin, Wyoming

Palynologic samples of the Fort Union Formation (Paleocene) were collected from selected composite measured sections and a drill hole in the Wind River Basin. Intervals studied are in outcrop in the Wind River Reservation in the Shotgun Butte area and near Hudson, Wyoming, and in the subsurface in the Muddy Ridge field. Age determinations were made using the palynostratigraphic zonation originally developed near Waltman, Wyoming, in the northeastern part of the basin. Although parts of the zonation have been called into question by other recent work in the basin, only the basal Paleocene biozone (P1) is in need of modification, and the zonation remains an effective tool for age determination and correlation in these rocks. Age control is sparse in parts of some sections studied, mostly because of a dominance of sandstone and conglomerate facies, but palynostratigraphy permits correlation of the study areas with each other and with the Waltman reference section. Palynology supports the following correlations: The lower unnamed member of the Fort Union Formation in the Shotgun Butte area correlates stratigraphically and chronologically with the lower part of the formation in the subsurface at Muddy Ridge field, with a short interval in the Hudson area, and with the lower and middle parts of the unnamed member in the Waltman area. The Shotgun Member correlates chronologically with the coal-bearing upper part of the Fort Union Formation in the subsurface at Muddy Ridge field and with the upper part of the unnamed member and Waltman Shale Member in the Waltman area. These correlations show that significant coal deposits in the subsurface have little or no expression in outcrop because of changes in facies.

Conference Paper↗
USGS1991

Shoreline deposits and diagenesis resulting from two Late Pleistocene highstands near +5 and +6 metres, Durban, South Africa

In exposures of Pleistocene rocks on the east coast of South Africa, eight sedimentary facies were distinguished on the basis of petrology, grain size, internal structures and field relationships. These are interpreted as deposits of surf zone, breaker zone, swash zone, backbeach, boulder beach and dune environments. Three phases of deposition and diagenesis are recognized. As a result of the stabilising effect of pre-existing coastal facies, the deposits from successive sea level stands are stacked vertically in a narrow coast-normal strip. Early cementation prevented erosion of the deposits during subsequent transgressions. Deposition of subsequent facies took place on an existing coastal dune (Facies 1). A terrace was cut into this dune at a sea level 4.5 to 5 m above present. At this sea level, clastic shoreline sediments were deposited which make up the main sedimentary sequence exposed (Facies 2–7). The steep swash zone, coarse grain size, and comparison with modern conditions in the study area indicate clastic deposition on a high-energy, wave-dominated, microtidal coastline. Vertical stacking of progressively shallower water facies indicates progradation associated with slightly regressive conditions, prior to stranding of the succession above sea level. During a subsequent transgression to 5.5 or 6 m above present sea level, a second terrace was cut across the existing facies, which by then were partly lithified. A boulder beach (Facies 8) deposited on this terrace is indicative of high wave energy and a rocky coastline, formed by existing cemented coastal facies. Comparison with dated deposits from other parts of the South African coast suggest a Late Pleistocene age for Facies 2–8. Deposition was terminated by subsequent regression and continuing low sea levels during the remainder of the Pleistocene. Cementation of the facies took place almost entirely by carbonate precipitation. The presence of isopachous fibrous cements suggests early cementation of Facies 1, 2, 3 and 4 under marine conditions, initially as aragonite which has since inverted to calcite. Facies 5, 6 and 7 are cemented only by equant calcite spar, evidence of cementation in the meteoric phreatic and vadose zones. Lowering of the water table during regression caused the remaining pore space in Facies 1, 2, 3 and 4 to be filled with equant calcite spar. Decementation in a 130 cm wide zone is attributed to water table shifts associated with the later transgression which deposited Facies 8. The vertical stacking of the two depositional sequences may be attributed to rapid cementation of Facies 2, 3, 4, 5, 6 and 7 under humid, subtropical conditions. This lithified sequence then acted as a focus for deposition of coarse-grained shoreline facies (Facies 8) during the subsequent transgression.

Marine Geology↗