ABSTRACT The Marias River canyon in north-central Montana, incised into Upper Cretaceous strata of the Great Plains during latest Pleistocene to Holocene time, served as a locus of human activity tied to the unique floral and faunal resources it provided ancient peoples. Erosion of the main canyon walls resulted in deposition of tributary junction alluvial fans characterized by debris-flow and hyperconcentrated flow sediment transport processes where side canyons emerged onto the alluvial valley floor. These alluvial-fan deposits preserve Late Precontact archaeological remains accessible due to their postburial exposure where partially eroded by the meandering channel of the Marias River (Goose Bill site complex). Archaeological materials are also preserved high on the dissected canyon walls where movement of hillslope sediment and colluvium by sheetwash led to their burial (Sparrowhawk site). Deposition of sediment of contrasting physical attributes (grain size, sorting, clay mineral content) within the canyon influences soil substrate properties, favoring growth of specialized plant communities in both alluvial-fan and fluvial environments. These relations exemplify the strong geoecological connections among depositional environment and sediment characteristics, substrate properties, and vegetation community development. The plant communities, interacting in conjunction with the physical landscape, provided a range of habitats utilized by such large mammals as plains bison, Rocky Mountain elk, Audubon sheep, pronghorn, Rocky Mountain mule deer, and white-tailed deer. Four major bison bone layers, some including bison hair and other soft tissues, with associated stone arrow points and stone flakes (artifacts) and evidence of bone processing are preserved in alluvial-fan deposits exposed along the bank of the Marias River at the Goose Bill site complex. These relations illustrate: (1) human activities dating to ~100-595 yr B.P. (falling within the Late Precontact Period of regional archaeological prehistory) that reflect subsistence reliance on bison as a food source, (2) the importance of tributary alluvial fans proximal to canyon walls in preserving archaeological remains, and (3) the role of the meandering Marias River channel in destroying these deposits over a time period of <100 yr. Sediment deposition by overland flow higher up the canyon walls at the Sparrowhawk site preserves a somewhat older (710-830 yr B.P.) archaeological record suggestive of a broader spectrum of resource (food) processing spatially decoupled from an area of bison kill/death.

Deposition of shallow-water carbonates on the vast Yangtze Platform of south China spanned the late Proterozoic through Middle Triassic, accumulating as much as 4000 m during the Early and Middle Triassic. Deeper-water carbonates and silici-clastics accumulated to comparable thickness in the Nanpanjiang Basin southeast of the platform. After the demise of the platform, an additional 2500 m of mostly silici-clastics spread across the platform in Late Triassic. Deposition of platform carbonates was also widespread in the Permian of south China, but a transgression in perhaps the last 2 m.y. of the Permian combined with differential subsidence to reconfigure the Yangtze Platform in the Triassic. The margin retreated ∼100 km northward in the Guiyang (eastern) sector, and a low-relief ramp developed over the flat top of the Permian platform, whereas the Early Triassic margin mimicked the Permian location in the Zhenfeng sector to the southwest. Nevertheless, deposition was essentially continuous from the Permian into the Triassic in most localities in Guizhou. Triassic deposition began with widespread terrigenous mud bearing thin-shelled bivalves or ammonoids. Renewed carbonate deposition within the Induan produced thin-bedded, laminated, dark-gray lime mudstone with planktonic biota in the basin interspersed with carbonate breccias. Thin-bedded lime mudstones with prominent burrows formed farther updip to the north and west. Thick intervals of oolite and of shallow-water lime mudstone interbedded with terrigenous clastic wedges that thicken and merge to the west mark the updip limit of deposition on a carbonate ramp. Platform-interior carbonates are more than three times as thick as their basinal equivalents. The ramp configuration evolved into a flat-topped platform with a slight rim in the Olenekian, recorded by peritidal carbonate cycles at the platform margin and subtidal lagoonal muds and ultimately evaporites in the interior. Carbonate deposition spread farther westward to cover the terrigenous siliciclastics of the Induan. A major deepening event within the Olenekian is marked by black, ammonoid-bearing, nodular limestone and fissile shale. The patchy distribution of this facies indicates differential warping of the platform rather than purely eustatic causes. The basin received a starvation diet of siliciclastic and carbonate mud with minor silty turbidites and carbonate debris flows. At the end of Early Triassic, platform-interior deposits averaged 1175 m thick and basinal equivalents only 250 m. Acidic volcanic ash spread across the Yangtze Platform at the Olenekian-Anisian transition. Anisian deposits in the Nanpanjiang Basin are dominantly siliciclastics that thicken dramatically in southwestern Guizhou and adjacent Guangxi. Biogenic frame-stones constructed by organisms of questionable origins, Tubiphytes and Plexoramea , assisted by sponges, arborescent corals, skeletal stromatolites, and copious encrusters, rimmed the Anisian platform. This rim collapsed along most of the Guiyang sector to form a basin-margin wedge 175 km long deposited by turbidity currents, debris flows, and rock fall. Collapse led to retreat of the margin by ∼2.7 km; exceptionally by 10 km. In contrast the reef margin advanced slightly in parts of the Zhenfeng sector. Mud-dominated peritidal cycles formed in the lee of the reefs. Deposition in the platform interior was entirely subtidal with alternating episodes of normal marine water, hypersalinity, and siliciclastic influx. An increase in siliciclastic content eastward throughout the Middle Triassic signals the emergence of the Jiangnan Massif, which had been covered throughout the Permian and at least the Induan. Platform sedimentation in the Ladinian features peritidal cycles that extended far into the interior to define a flat-topped platform. High depositional energy is reflected in grainstones and packstones composed of grapestone, bioclasts, and ooids. Barriers other than sand shoals appear to have been absent. Biogenic facies are confined to small outcrops of Tubiphytes and coral boundstone, interpreted as patch reefs. Tepee structures cap many cycles or disrupt successive cycles, indicating extended subaerial exposure. At the beginning of the Ladinian, the platform margin of the Guiyang sector prograded at least 0.6 km. In the Zhenfeng sector the margin retreated slightly, further indication of more rapid subsidence of the western part of the platform. The Nanpanjiang Basin apparently starved early in the Ladinian, but filled to overflowing with siliciclastic turbidites and mud in the later Ladinian or the early Carnian. Transport direction of the very fine sand of the turbidites was from the east, pointing to the Jiangnan Massif as a continued source of sediment, both by land onto the platform and by sea into the basin. East and west sectors of the Yangtze Platform in Guizhou present stark contrasts during the Carnian and Norian. Shallow-water carbonate deposition continued into the Carnian in the Guiyang sector, but tongues of terrigenous mud and sand from the northeast reached to the platform margin and damped out carbonate deposition by the end of the Carnian. Erosion prevailed in the Norian, truncating formations toward the north down to the level of the Anisian. Shallow-water carbonate deposition ended dramatically with the beginning of the Carnian in the Zhenfeng sector. Nodular-bedded, dark-gray lime mudstone with ammonoids overlies peritidal deposits, documenting the drowning of the Yangtze Platform. A very condensed sequence of black shale with concentrations of manganese, reduced iron, and organic carbon followed. Siliciclastic flysch and shallow-water sandstone totaling 1265 m thick filled the accommodation space resulting from the drowning by the end of the Carnian. Norian deposits are cross-bedded sandstones and conglomerates that form thinning- and fining-upward cycles attributed to braided streams that encroached on coastal swamps represented by commercial coal and by mudrocks containing fresh-water, brackish, or marine fossils. The braided streams were rejuvenated and apparently reversed in the Rhaetian to form a coarse-grained clastic wedge that thins and fines toward the north and east across an erosion surface that cuts as deep as Anisian rocks in northern Guizhou. Jurassic and Lower Cretaceous rocks overlie the Rhaetian rocks concordantly, contradicting prevalent interpretations of a major orogeny (Indosinian) in the Late Triassic in Guizhou. The angular unconformity underlying Upper Cretaceous conglomerates dates major deformation in Guizhou as mid-Cretaceous.

Abstract The accumulation of organic matter in depositional environments is controlled by complex, nonlinear interactions of three main variables: rates of production, destruction, and dilution. Significant accumulations of organic-matter-rich sediments can arise from many combinations of these factors. Although a few organic accumulations are dominated by one or another of these factors, most organic-matter-rich sediments and rocks record a variety of optimized interactions of all variables. The Mowry Shale (Cretaceous, Western Interior, USA) illustrates a transition from dilution-dominated organic accumulation to production-driven accumulation along a 450-km-long onshore–offshore transect. Production-related variations in organic-matter content appear to be driven by the disparate relative rates of population growth of primary producers ( r- selected opportunists) and consuming organisms ( K -selected specialists). Peak organic-matter enrichment occurs in the uppermost transgressive systems tract in proximal areas and in the lower highstand systems tract in distal reaches. Organic-matter enrichment in the upper Brushy Canyon and lower Cherry Canyon Formations (Permian, west Texas) arises from local optima of burial rates and pelagic organic-matter input. Significantly enriched strata occur in siltstones (with minimal clay contents) interbedded with slope and basin-floor sandstones deposited under oxic to suboxic conditions. Enrichment is not strongly correlated with either oxygen deficiency or primary production, and shelf-derived organic matter is of minimal importance. Rather, enrichment appears to be largely a function of preservation by burial at optimal net sediment accumulation rates. The portions of the Monterey Formation (Miocene, California) most enriched in organic matter paradoxically do not represent the intervals of highest organic-matter production. Highest production rates dilute organic matter by other biogenic material and correspond to highly siliceous lithologies (cherts, porcelanites). Good preservational conditions and moderate primary production rates appear to control significant concentrations of organic matter. Conceptually, organic-matter enrichment can be expressed as an overall simple relation that is quite complex in detail because of the interdependencies of the variables: Organic-matter enrichment = Production – (Destruction + Dilution) where: Production = f (Nutrient supply), Destruction = f (Production of organic matter) + f (Oxidant exposure time) – f (Clastic sedimentation rate < burial-efficiency threshold), and Dilution = f (Clastic sedimentation rate > burial-efficiency threshold) + f (Production of biogenic silica or carbonate). Significant enrichment of organic matter occurs where organic-matter production is maximized, destruction is minimized, and dilution by clastic or biogenic material is optimized. Hence there are a range of depositional settings that accumulate source rocks.

Abstract Understanding of the incompatibility of King’s three-fold classification of the Proterozoic (‘Purana’) rocks of the Pranhita–Godavari Valley, south India, and renewed mapping and stratigraphic studies, reveal that syn-depositional basin dynamics, with differential uplift and subsidence, was the major control on basin evolution and the development of multiple unconformity-bound successions. The unconformities demarcate six major successions, the Mallampalli/Devalmari, Somanpalli, Mulug, Penganga, Albaka and Sullavai groups, each with a very distinctive set of sedimentological attributes. There are at least five different conflicting classification schemes so far. The major problems in the classification of the succession appear to lie in the recognition of unconformities and unconformity-bound successions, and in stratigraphic nomenclature. Based on field geology, available age data and the concept of the megasequence, the Mallampali, Devalmari, Mulug and Somanpalli groups have been grouped into the Pakhal Supergroup. The question of combining the rest of the groups has been deferred until the resolution of the Albaka–Penganga relationship is achieved beyond doubt. The relationships between different stratigraphic units have been critically examined to reconstruct the stratigraphic history, events of sea-level change and palaeogeographical evolution.