Abstract Originating in the Himalayan Mountains within distinct drainage basins, the Ganges and Brahmaputra rivers coalesce in the Bengal Basin in Bangladesh, where they form one of the world’s great deltas. The delta has extensive subaerial and subaqueous expression, and this paper summarizes the current knowledge of the Late Glacial to Holocene sedimentation from the upper delta plain to the continental shelf break. Sedimentation patterns in the subaerial delta are strongly influenced by tectonics, which has compartmentalized the landscape into a mosaic of subsiding basins and uplifted Holocene and Pleistocene terraces. The Holocene evolution of the delta also has been mediated by changing river discharge, basin filling, and delta-lobe migration. Offshore, a large subaqueous delta is prograding seaward across the shelf, and is intersected in the west by a major submarine canyon which acts both as a barrier for the further westward transport of the rivers’ sediment and as a sink for about a third of the rivers’ sediment discharge. Subaerial and subaqueous progradation during the Holocene has produced a compound clinoform, a feature which appears to be common for large rivers discharging onto an energetic continental shelf.

Abstract A putative Precambrian paleosol mapped at the unconformity between the Cambrian Flathead Sandstone and Belt Supergroup at Fishtrap Lake, Montana, was found instead to be a succession of paleosols forming the basal portion of the Flathead Sandstone. Early Cambrian age of these paleosols comes from stratigraphic ranges of associated marine trace fossils: Bergaueria hemispherica, Didymaulichnus lyelli, Torrowangea sp. indet., and Manykodes pedum . Instead of a single strongly developed paleosol on top of the Belt Supergroup with a smooth geochemical depth function, five successive geochemical and petrographic spikes were interpreted as so many individual paleosols within a short sedimentary sequence of red beds, overlying brecciated and little-weathered Belt Supergroup. The most weathered intervals (paleosol A horizons) are purple-red in color (Munsell weak red, 7.5R 4/2) and massive to hackly, whereas intervening marine siltstones are planar bedded and purple-gray (Munsell dark reddish gray, 7.5R 4/1). The massive to hackly appearance comes from blocky to platy peds defined by argillans and is also the result of pervasive bioturbation of two distinct kinds: drab-haloed filament traces and ferruginized-organic filaments. In thin section, the filaments are circular as well as elliptical and elongate and of presumed microbial origin. The filament-rich (A) horizons are also defined by magnetic susceptibility and show petrographic evidence of significant weathering (depleted abundance of rock fragments, feldspar, and mica compared with lower horizons). Additional evidence of weathering comes from chemical analyses showing net loss of mass and weatherable elements within a profile. These lines of evidence indicate that Montana estuarine landscapes during the earliest Cambrian were colonized by filamentous organisms in a tropical humid paleoclimate, rather than the frigid conditions documented elsewhere during the Late Ediacaran and Early Cambrian.