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.

Abstract Observations and data from modern streams and recent deposits demonstrate that river systems in subhumid and semiarid seasonal settings in tropical and subtropical latitudes can have deposit characteristics very different from those predicted from previously published facies models based on other climatic settings. The differences result primarily from the extremely variable discharges that are typical of these climatic settings and contribute to a distinct fluvial style. Five critically important characteristics of the deposits are illustrated by data from NE Australia. The deposits with these characteristics occur in streams with variable discharge and are independent of channel size. (1) The channel-fill lithosomes are erosionally based and exhibit complex lateral facies changes. (2) Within the channel-fill lithosomes there are abundant mud partings, some of which are pedogenically modified. (3) In most cases the complex internal architecture lacks the macroform elements, such as lateral-accretion cross bedding, typical of other fluvial sediment bodies. This is the case even within point bars. (4) Sedimentary structures formed under high flow stage are abundant and frequently preserved in the lithosomes. (5) Trees and other vegetation adapted to occasional inundation by fast-flowing water may colonize channel floors (not only on banks), and these both generate organic sediment and influence the flow and resulting sediment deposition patterns. In addition to the characteristics of the channel-fill deposits, the overbank deposits may have diagnostic character and the sand petrography may differ from that in other settings. The same features are identified in the rock record and a detailed case study from the Pennsylvanian of the Maritimes Basin Complex of Atlantic Canada illustrates the distinction between the seasonal tropical and other fluvial styles. The upper Namurian (Yeadonian) Boss Point Formation shows a transition from perennial to subhumid to semiarid, seasonal tropical fluvial styles within a succession of sheet-like braidplain channel bodies. The overlying, basal Westphalian (Langsettian), Little River Formation is composed entirely of more lensoid channel bodies displaying the subhumid to semiarid, seasonal tropical fluvial style, and associated overbank deposits. The overlying coal-bearing Joggins Formation shows a gradual return to a perennial fluvial style with channel body geometries similar to the underlying unit. A clear signal of paleoclimate change can be deconvolved from variations in accommodation regime, providing a hitherto unavailable discriminant for interpreting fluvial successions. Recognition of the strongly seasonal, tropical to subtropical fluvial style in the rock record, and of changes in character through vertical successions, will aid paleoclimate interpretation and subsurface reservoir analysis in fluvial successions.

Abstract Criteria for identifying alluvial-valley fills in the geological record have commonly been derived from coastal areas, where high-magnitude glacioeustatic fluctuations generate predictable systems tracts. In contrast, inland alluvial valleys formed in response to climatic, tectonic, and intrinsic events of varied magnitude and duration. Sequence boundaries and architecture reflect a variety of causes, typically mediated through changes in the river’s equilibrium profile and equilibrium floodplain height. We evaluate valley and sequence models using Quaternary examples from southern Asia and Australia. For these areas, proxy records are well established for climate and sea level, but the scarcity of records for tectonic activity and intrinsic channel changes hamper an interpretation of stratigraphic events. On the large alluvial plains of northern India and central Australia, climatic changes have radically affected sediment and water discharge on timescales as short as 10 3 to 10 4 years. Periods of incision and aggradation generate discontinuity-bounded sequences (aggradational–degradational rhythms), and reaches may vary between valley and channel morphology over short periods and distances. Across Asia, changes in monsoonal precipitation have caused near-synchronous incision and aggradation in varied tectonic settings. However, mature interfluve paleosols correlative with valley bases may be local or absent on these large plains, where small interfluve channels, eolian dunes, and lakes aggrade while mainstem valleys incise. Gullied floodplains, adjacent to valleys and tributaries, and terrace deposits are useful indicators of incised systems. Climatic effects appear to be underrepresented in interpretations of ancient alluvial sequences in general. Angular unconformities, channel diversion, and incised and tilted fans near major faults reflect tectonic controls on architecture, and soft-sediment deformation and rare surface ruptures testify to earthquake events. In smaller basins such as the Rio Grande Rift of southwestern USA, alluvial architecture reflects both tectonic and climatic controls. Channel-body amalgamation may not be a good proxy for tectonically induced subsidence, especially for megafans at upland exits where a low degree of amalgamation may reflect rapid accumulation rather than rapid subsidence and accommodation creation. Multistory valleys and extensive multistory and single-story sheets are present in inland and coastal alluvial areas. However, below the Australian continental shelf, seismic profiles identify entrenched channels and channel belts rather than the valley fills that are commonly assumed to form under such conditions. This example shows that sea-level fall and river extensions need not generate deeply incised fluvial valleys in coastal settings.