Bituminous coals on emergent surfaces in an Asbian, lower Carboniferous (Mississippian) limestone succession on the North Wales carbonate platform, UK, and implications for palaeoclimate

Emergent surfaces in the Mississippian (Asbian to Brigantian) carbonate platform succession of North Wales record periods of plant colonisation and peat formation that led ultimately to the local development of coals. Examination of bituminous coals on three emergent surfaces within Cefn Mawr Quarry reveals information on palaeoclimate that is not available from study of the limestones alone. Three coal seams in the Asbian Loggerheads Limestone Formation were identified and the lowest one studied in detail. Vitrinite reflectance data from alternating bands of vitrite and duroclarite microlithotypes, the distribution of pyrite within them, and the sharp contacts between them, suggest that there were abrupt changes in marine influence during the development of the peats that formed the coals. It is inferred that local palaeoclimate alternated between periods of high and low rainfall, the amount of rainfall influencing the extent to which seawater encroached into the peats, with higher rainfall suppressing the ingress of saline waters into groundwater. On the basis of modern peat growth rates, the timescale of the alternation indicated by each duroclarite-vitrite couplet is suggestive of an annual cycle, such as would arise in a monsoonal climate. The low proportion of ash in the three coals, the preservation of internal lamination, the low diversity of spore species in the lowest coal compared with the over- and underlying mudrock, and the presence of rhizoconcretions in palaeokarstic limestone beneath the lowest and highest coals, demonstrate that the peat swamps were isolated from the hinterland and autochthonous. This study demonstrates that a wider application of palynology and coal petrology is an important contribution to the study of marine carbonate successions of any age where terrestrial organic matter, formed during emergence, has been preserved.

Mississippian Chat and Tripolite Zones in Osage County, Oklahoma: Paleokarst Interpretation based on 3D Seismic and Well Logs

Mississippian paleokarst Chat and tripolitic chert (tripolite) zones associated with the Mississippian Lime have been hydrocarbon exploration targets in Osage County for many decades. Chat is residual chert, either in place or transported, weathered out of chert-bearing Mississippian Limestone that was eroded at the Mississippian-Pennsylvanian unconformity. Thus, the formation of Chat is an epigene paleokarst process. Tripolite occurs as a highly porous, silica-rich interval within the Mississippi Lime. It is formed by in-place alteration of limestone by silica-rich surface waters or deep-seated hydrothermal fluids, making tripolite formation a mixed or hypogene paleokarst process. Here, we have studied Chat and tripolite by seismic analysis calibrated by well control with full-wave sonic and density log data. We identify that Chat and tripolite show clear separation from Mississippian Lime log-based acoustic impedance and VP/ VS, but there is no meaningful separation of Chat from tripolite, and they both exhibit total porosities greater than 20% with evidence of fracture porosity. We find that the sonic-based normal incidence wedge model for Chat bounded above by the Pennsylvanian Shale and below by the Mississippian Lime indicate that two seismic expressions are plausible: first, a strong negative amplitude when the Chat thickness is above the tuning thickness (for this survey it is 56 ft) and, second, a weak positive or negative amplitude associated with the small impedance contrast between Chat and overlying Pennsylvanian Shale. Our analysis suggests that the traditional Chat “strong response” and a new “dim-out” exploration strategy may be usefully applied in Osage County. We show that the tripolite response is consistently a negative amplitude event that strengthens with increasing tripolite thickness. We provide an interpretive framework for characterizing Chat and tripolite zones associated with the Mississippian Lime in the US Midcontinent, which may apply to regions around the world.

Late Mississippian limestone sedimentary environment in southern Pembrokeshire (Bullslaughter Bay, Wales): evidence of meteoric diagenesis and hypersaline features

Bullslaughter Bay in southern Pembrokeshire, UK, exposes sections of Upper Mississippian limestone strata. In many places, the rock suffered an isovolumetric alteration during a period of sea-level oscillations. We used multiple approaches to study the weathered rocks, combining sedimentological, petrographic and isotopic compositions (δ18O and δ13C values). Two main microfacies are recognized: (i) packstones/grainstones, characteristic of an open marine shallow subtidal/intertidal environment, with a high degree of agitation, slightly elevated salinity and temporary subaerial exposure; and (ii) mudstones/wackestones in a lagoonal setting and intertidal or supratidal environments, with a pedogenetic influence. In both cases, a complex diagenetic story, which started early in a meteoric environment, induced a strong alteration producing loose sediments in place of the parent rock. Calcretization, at or near the sediment surface in the vadose zone, was one of the most widespread diagenetic modes. It could be associated with beachrocks. Carbon and oxygen stable isotope analyses from more or less weathered limestones support the petrographic data: they show non-marine values with δ13C ranges of from −2.13 ‰ to 1.75 ‰ and δ18O from −6.05 ‰ to −4.66 ‰. These values are systematically lower than those of the middle Carboniferous seawater. Some periods of low sea level and subaerial exposure allowed gypsum to form. Neoformation of euhedral quartz by probable replacement after sulfate, and halite pseudomorphs after gypsum in a hypersaline environment are documented for the first time in southern Pembrokeshire. The studied weathered limestones present a complex diagenetic evolution related to sea-level oscillations in a range of hot and contrasting seasonal climates.

Quantitative analysis of facies variation using ground-based lidar and hyperspectral imaging in Mississippian limestone outcrop near Jane, Missouri

Ground-based hyperspectral imaging is useful for geologic mapping because of its high spectral and spatial resolutions at a millimeter to centimeter scale. We have used hyperspectral and terrestrial laser scanner (TLS) data collected in close range to a roadcut near Jane, Missouri, that contains a subvertical outcrop of Lower Mississippian limestone. The outcrop consists of the Compton, Northview, and Pierson Formations, which we evaluated for facies heterogeneity. The sequence near Jane, Missouri, was deposited in shelf margin with high-frequency sea-level fluctuations. These fluctuations introduced lithologic and geometric heterogeneity to the facies, and debris flows brought in carbonate mounds referred to as outrunner blocks. These are important to interpret accurately because of their equivocal depositional origin, which is highly debated by previous workers. We combined hyperspectral data with TLS for an integrated spatial analysis of geometric and compositional variations in facies by accurate, point cloud-registered mineralogical mapping. We mapped several carbonate facies based on spectral signatures of calcite, silt, and clay particles and distinguished pure limestone outrunner blocks from surrounding mud-prone limestone facies with various proportions of silt and clay (a total of approximately 60%). By tracing the classified facies from combined hyperspectral and TLS imagery, we produced a lithostratigraphic framework, which indicates rapid changes in lithology and the presence of shale baffles that vary the character of the Compton through Pierson interval and contribute to heterogeneity in this outcrop. The data suggest a lower energy depositional environment and support the hypothesis of transported outrunner blocks in a distally steepened ramp system. The information that we have evaluated in our study could help to explain reservoir heterogeneity in equivalent carbonate fields.