Cryptic sequence boundaries in braided fluvial successions

In braided fluvial deposits, consisting of monotonous successions of sandstone or conglomerate, it may be difficult to distinguish regionally significant bounding surfaces (sequence boundaries) from autogenic channel-scour surfaces. Major surfaces may be characterized by erosional relief and draped by lag deposits, but not all sequence boundaries show these characteristics. Other clues to the presence of a major surface are sharp changes in detrital composition, shifts in regional palaeocurrent trends and evidence of early diagenesis of the sandstones immediately below the sequence boundary. Examples of these attributes of cryptic sequence boundaries are illustrated from three Mesozoic units in the Colorado Plateau area of the United States. In the Chinle Formation (Triassic), near Moab, Utah, angular intraformational unconformities overlie sandstones showing evidence of early diagenesis. In the Castlegate Sandstone (Upper Cretaceous) of east-central Utah, a cryptic sequence boundary can be discriminated from other erosion surfaces by the evidence of detrital petrography and early diagenesis. Palaeocurrent data indicate changes in regional palaeoslope at two sequence boundaries within this unit. Evidence of early diagenesis is also present at a sequence boundary in the Kayenta Formation (Jurassic) of westernmost Colorado.     

Whither stratigraphy?

There have been three revolutions in sedimentary geology. The first two began in the 1960s, consisting of the development of process-response sedimentary models and the application of plate-tectonic concepts to large-scale aspects of basin analysis. The third revolution, that of sequence stratigraphy, began in the late 1970s and helped to draw together the main results of the first two: the knowledge of autogenic processes learned through facies analysis, and the understanding of tectonism implicit in the unravelling of regional plate kinematics. Developments in the use of seismic-reflection data and the evaluation of a hypothesis of global eustasy provided considerable stimulation for stratigraphic research.Current developments in the field of sequence stratigraphy are focusing on three areas. (1) Elaboration of the sequence-architecture models for various configurations of depositional environment and sea-level history. (2) Exploration of various mechanisms for sequence generation, especially tectonism and orbital forcing. (3) Attempts to improve the level of precision in stratigraphic correlation and to refine the geological time scale, as a means to test the model of global eustasy.The growth in the power of computers and our knowledge of physical and chemical processes has led to the evolution of an entirely new way of evaluating earth history, termed quantitative dynamic stratigraphy. Mathematical modelling and numerical simulation of complex earth processes are now possible, and require the collection and integration of a wide array of quantitative and qualitative data sets. Applications include the study of the geodynamic evolution of sedimentary basins, modelling of stratigraphic sequences and global climates, studies of Milankovitch cycles (cyclostratigraphy) and simulation of fluid flow through porous media. The Global Sedimentary Geology Program has brought many of these areas of study together in multidisciplinary, global-scale studies of the sedimentary history of the earth. The results of these studies have wide application to many problems of importance to the human condition, including the past history of global climate change and other environmental concerns. The study of stratigraphy is at the centre of the new view of the earth, termed earth-systems science, which views earth as an ‘organic’ interaction between the lithosphere, biosphere, hydrosphere, and atmosphere.     

Concept and styles of reciprocal stratigraphies: Western Canada foreland system

The late Cretaceous–Palaeocene Western Canada foreland system provides a record of high-frequency cycles of reciprocal flexural tectonics superimposed on longer term changing dynamic subsidence. Initial dominance of dynamic subsidence during the deposition of the Bearpaw marine succession resulted in continual subsidence with differential rates across the flexural hinge line. Subsequent dominance of flexural tectonics resulted in opposite base-level changes between the proximal and distal reaches of the foreland system during the deposition of the post-Bearpaw nonmarine succession. In both cases, the contrasting base-level changes generated out-of-phase stratigraphic sequences, which defines the concept of reciprocal stratigraphies. Two styles of reciprocal stratigraphies have been identified in relation to the pattern of base-level changes across the foreland system. The Bearpaw style consists of a conformable succession of correlative transgressive and regressive systems tracts, suggesting continuous basin-wide sedimentation with the rates within the range of variation of the rates of base-level rise. The post-Bearpaw style shows sequences correlative to age-equivalent sequence boundaries related to coeval rising and falling base-level, respectively. The succession of Bearpaw and post-Bearpaw sequences corresponds to a cycle of marine to nonmarine foreland sedimentation controlled by changing ratios between dynamic subsidence and flexural tectonics. The amount of sediment supply derived from the orogen to the foreland system may also reflect the rates of dynamic subsidence, as a decrease in dynamic loading may lead to accelerated denudation of the sediment source areas.     

The Siluro-Devonian clastic wedge of Somerset Island,Arctic Canada,and some regional paleogeographic implications

Upper Silurian and Lower Devonian rocks on Somerset Island were derived from Boothia Uplift, a linear tectonic belt that underwent a pronounced period of positive movement commencing in the Pridolian. The lower part of the clastic wedge is a succession of predominantly intertidal to supratidal dolomite and siltstone 150–400 m thick (Somerset Island Formation). Markov analysis documents the presence of tidal cycles in these rocks. The succession changes eastward, away from the uplift, into subtidal marine limestone indistinguishable from that of the underlying Read Bay Formation. The Somerset Island Formation grades vertically and laterally westward into alluvial deposits of the Peel Sound Formation, which consists of red sandstoné of braided river and possibly eolian origin, and two fanglomerate members. A variety of fluvial fining-upward and thinning-upward cycle types has been documented by Markov analysis of a lower sand member of the formation. The Peel Sound reaches a maximum thickness of 600 m in northern Somerset Island.Paleocurrent analysis of crossbedding indicates eastward transport directions in the fluvial rocks, except for cosets of very large scale crossbeds (up to 6 m thick) in the northwestern part of the island, which indicate northwesterly flow. The large sets are interpreted as the deposits of eolian dunes, or of sand waves in a large trunk river which carried clastic detritus northward, parallel to Boothia Uplift.The Peel Sound Formation and its lateral facies equivalents on Prince of Wales Island, west of Boothia Uplift, contain coarser conglomerates and a higher sandstone/carbonate ratio, indicating deposition under higher energy conditions than are thought to have prevailed in Somerset Island. Relief may have been greater and depositional slope steeper in the west, an asymmetry in Boothia Uplift that is reflected in the present-day structural style of a narrow zone of tilting and reverse faulting in the west, and a broad zone of gentle folding and normal faulting in the east. The similarity in structural and stratigraphic asymmetry indicates a genetic link, which is further suggested by the presence of syndepositional folds and unconformities at a few localities within the clastic wedge.An estimate of the volume of sediment removed from Boothia Uplift indicates that approximately one third can be accounted for in the present clastic wedge on either side of the Uplift. Either the Lower Paleozoic formations were attenuated over the Uplift, or rivers such as the hypothetical trunk river were effective in the dispersal of material beyond the region of the clastic wedge.