ABSTRACT Few detailed diagenetic studies have been carried out on the Mississippian limestone of north-central Oklahoma, U.S.A. Facies analysis, petrographic observations, and stable isotope data are integrated to investigate diagenetic history. This progradational succession of heterozoan–biosiliceous carbonates accumulated on the southern margin of the Burlington–Anadarko shelf. Diagenetic products related to mesogenesis are pervasive, whereas those related to eogenesis and hydrothermal alteration are localized. A pervasive burial diagenetic overprint is consistent with patterns in stable isotopic data, the bulk of which define a trend of large decreases in δ 18 O ( − 1.5 ‰ to − 7.5 ‰ ) accompanied by relatively small decreases in δ 13 C ( + 3.5 ‰ to + 1.5 ‰ ) values. Microbioclastic skeletal wackestone–packstones are prominent. Due to low diagenetic potential, these facies entered the burial realm essentially unlithified. They are characterized by features that point to chemical compaction as the primary driver for lithification. Spicule-rich facies experienced a different diagenetic pathway, with silicification leading to lithification prior to physical compaction. Late-stage fracturing and hydrothermal alteration, significant elsewhere in the basin, is only locally developed. Porosity is associated largely with late-stage fractures and solution seams resulting from chemical compaction.

Abstract: This study investigates the Neogene strata of the AND-2A core recovered by the ANDRILL–Southern McMurdo Sound Project in the Victoria Land Basin, Antarctica, as an analog for assessing controls on reservoir quality in glacimarine deposits. The succession comprises a series of depositional sequences formed in marine environments within a failed rift under the influence of repeated advances and retreats of glacial ice, with attendant changes in sea level and sediment supply. Stratal cycles (sequences) typically follow a vertical succession from a basal diamictite deposited in ice-proximal settings fining upward into shoreface sandstone, muddy sandstone, and mudrock. The fining-upward sequence then coarsens upward into coastal and nearshore muddy sandstones and sandstones. Changes in paleoclimate mode through the Neogene caused variations in sequence development, including changes in sequence thickness, variety and range of facies, changes in the completeness of sequences, and changes in the proportion and character of diamictites. Results show that reservoir quality in glacimarine sandstone is dramatically affected by the presence of diagenetic carbonate precipitated during burial from connate cryogenic brine. Strong correlations exist between carbonate cement abundance, paleoclimate, and sequence stratigraphic systems tract. Sandstones that formed during the coldest (polar and subpolar) climate regimes have relatively low porosities (<15%) due to occlusion of pore space by carbonate cement. Decreased production and deposition of mud-sized material during the coldest climate conditions produced sequences characterized by higher overall permeability that were prone to infiltration by brine upon burial, leading to cementation. By contrast, the sandstones formed during relatively temperate climate regimes preserve higher porosities (25–45%) and lack significant cementation. Sequence stratigraphic relationships indicate that these porous sandstones are best developed in highstand delta systems that formed during ice minima. Individual sandstone bodies, which extend laterally over several kilometers, are enclosed by muddy lithologies. Porosity in these sandstones was retained as a result of discharge of dilute meltwater during deposition and subsequent isolation of sands between impermeable barriers. Trends identified in this study may prove useful in predicting and locating target reservoirs in other glaciogenic and glacimarine settings worldwide.

Abstract: Criteria for recognizing a high-paleolatitude context for sedimentary successions are not widely established. Herein, we provide a facies analysis of the Permian succession of the high-paleolatitude Denison Trough in the southwestern Bowen Basin of Queensland, eastern Australia, and we use this analysis to highlight criteria that may be used to diagnose a high-paleolatitude context in this and other successions. A unified facies scheme for several formations, combining sedimentological and ichnological criteria, recognizes both deltaic and nondeltaic facies within the succession. Whereas a full array of deltaic facies is evident, ranging from distal prodelta to coastal plain, a more limited array of nondeltaic facies is recognized, ranging from shelfal to lower shoreface. The dominance of deltaic facies in the succession suggests that coastlines were overwhelmingly deltaic in aspect. The absence of middle and upper (nondeltaic) shoreface deposits suggests that shallow-water settings were constantly under physico–chemical stresses associated with deltaic efflux, and/or that such deposits were excised by transgressive ravinement following deposition. Deltas were mostly arcuate in planform, consistent with strong wave influence, although some show a more irregular or lobate plan morphology, suggesting significant fluvial influence. Four intervals within the Permian succession (coded P1 to P4) preserve evidence of formation under the direct or indirect (glaciomarine) influence of glacial ice. Palpable evidence of the high-paleolatitude context of the succession is preserved only in these intervals, most commonly in the form of dropstones, glendonite pseudomorphs after ikaite, gravel-grade clasts with modified shapes, and diamictites. In addition to vertical changes into and out of glacial intervals, paleolatitudinal changes in glacially influenced facies are evident across the 25- to 30-degree meridional transect from the Bowen Basin south to the Tasmanian Basin. Outside of glacial intervals P1 to P4, there are few sedimentological or ichnological indicators of high-paleolatitude deposition. Facies characteristics of deposition under glacial influence are therefore crucial to diagnosing the high-paleolatitudinal context of this and other successions.

Abstract This paper provides documentation of unexpectedly high-reservoir-quality glaciomarine sands found in the Cenozoic succession beneath McMurdo Sound, Antarctica, as an analogue study for evaluations of hydrocarbon prospectivity in basins elsewhere. The Oligocene to Lower Miocene succession of the Victoria Land Basin, an extant portion of the West Antarctic Rift System, comprises diamictites, mudrocks and sandstones with minor conglomerates. These lithologies are arranged in repetitive stacking patterns (cycles), interpreted to record repeated advance and retreat of glaciers into and out of the basin, with attendant eustatic and isostatic effects. Phases of ice retreat within the cycles comprise an array of mudrocks, sandy mudrocks and sandstones, deposited mainly during relative sea-level highstands. Clean, well-sorted, unconsolidated and porous sands <25 m thick from such intervals, which are interpreted to be mainly deltaic in origin, were encountered. Some of these sands, which have visible porosity as high as 41%, flowed into the well bore together with significant volumes of cold formation water. Diagenetic modification of sands in these intervals is minimal, which can be attributed to the low-temperature nature of the subsurface environment. Accordingly, glaciomarine sands in near-field glaciogenic successions should be considered as potential reservoir facies in prospectivity assessments.

No abstract available.

Gondwana, though extant for approximately one-half billion years, is now present as fragments across much of the globe. Following an assembly during the latest Protero zoic into the early Phanerozoic, the megacontinent has gradually fragmented to its current dispersed pattern. Paleozoic fragmentation, primarily on its north and west margins, formed a series of ribbon-shaped continents that collided with southern Laurasia and generated major orogenic events. Meanwhile, much of its southern and eastern margin was the site of subduction and associated Cordilleran-style tectonics. Mesozoic and Paleogene rifting completed the fragmentation, sending continents northward to generate the Alpine-Himalayan mountain chain from Spain to China. Much of Gondwana flirted with the South Pole throughout the Paleozoic, and several major glacial episodes resulted. The largest and most extensive of these was the late Paleozoic ice age; the consequences of this event dominated global geology for nearly 100 m.y. and orchestrated the greatest cyclic stratigraphic record in Phanerozoic history.