A quantitative, three-dimensional depositional model of gravelly braided rivers

摘要

A quantitative, three-dimensional depositional model of gravelly, braided rivers has been developed based largely on the deposits of the Sagavanirktok River in northern Alaska. These deposits were described using cores, wireline logs, trenches and ground-penetrating radar profiles. The origin of the deposits was inferred from observations of: (1) channel and bar formation and migration and channel filling, interpreted from aerial photographs; (2) water flow during floods; and (3) the topography and texture of the river bed at low-flow stage. This depositional model quantitatively represents the geometry of the different scales of strataset, the spatial relationships among them and their sediment texture distribution. Porosity and permeability in the model are related to sediment texture. The geometry of a particular type and scale of strataset is related to the geometry and migration of the bedform type (e.g. ripples, dunes, bedload sheets, bars) associated with deposition of the strataset. In particular, the length-to-thickness ratio of stratasets is similar to the wavelength-to-height ratio of associated bedforms. Furthermore, the wavelength and height of bedforms such as dunes and bars are related to channel depth and width. Therefore, the thickness of a particular scale of strataset (i.e. medium-scale cross-sets and large-scale sets of inclined strata) will vary with river dimensions. These relationships between the dimensions of stratasets, bedforms and channels mean that this depositional model can be applied to other gravelly fluvial deposits. The depositional model can be used to interpret the origin of ancient gravelly fluvial deposits and to aid in the characterization of gravelly fluvial aquifers and hydrocarbon reservoirs.