Galloway (1981) concluded that the depositional architecture of aggrading coastal plain fluvial sequences includes both the three-dimensional interrelationships and geometries of component framework sand bodies and the distribution of bedding and internal structures within the framework facies. Examination of late Quaternary and Oligo-Miocene fluvial systems of the Texas Coastal Plain illustrates the architectural variety inherent in aggradational coastal plain sequences and indicates common depositional styles:
I. Coastal plain sequences typically record deposition by multiple synchronous fluvial systems, ranging from large extrabasinal rivers to small streams draining the basin fringe and local intrabasinal streams. Identification and definition of the major systems and their component depositional elements (fluvial axes and interaxiai streamplains,
lacustrine basins, or aeolian plains) is the initial step in reconstruction of basin paleogeography.
2. Contemporaneous major as well as minor rivers may possess grossly differing load, discharge, and geometric characteristics. Consequently, deposits of each depositional element commonly differ greatly in terms of internal structure, mineralogical and textural composition, and sand-body geometry.
3. Major extra basinal rivers typically enter the aggradational coastal plain at topographically or structurally stabilized points. Once on the coastal plain, the river will radiate from its point source in an attempt to aggrade the depositional surface as evenly as possible. Both nodal and random down-channel avulsions result in successive
occupation of multiple channel trends. Facies maps of thick sequences of channel and associated deposits (i.e., sand isolith or percent maps) consequently show complex interweaving or distributing trends. Lateral spacing of major rivers determines the along-strike extent of distributary alluvial aprons. Long-term stability of the loci of sediment input favors vertical persistence of fluvial elements and systems.
4. Within fluvial axes, framework channel facies may form multilateral belts up to tens of miles in width, or may stack vertically. Dry climate and dominance of bed load or extremely flashy fluvial systems favor development of multilateral sand belts. Mixed-load fluvial systems are characterized by moderate to strong vertical stacking of framework sand bodies.
5. Significant vertical change in depositional style within major fluvial axes records the evolution of river pattern in response to extrinsic or intrinsic changes in the drainage or depositional basin. In the Texas Coastal Plain, mixed-load fluvial axes commonly exhibit evidence for upward decrease in river bed-load content.
Because fluvially deposited stratigraphic units peripheral to large marine sedimentary basins most likely consist of a mosaic of individual depositional systems and component elements, accurate description and interpretation of coastal plain paleogeography necessitates a hierarchical, three-dimensional approach to analysis of alluvial stratigraphy.
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