Depositional Trends of Modern and Ancient Deep-Sea Fans
Many flysch, turbidite, fluxoturbidite, and grain-flow sequences from ancient geosynclines probably have been deposited in deep-sea fans adjacent to continental margins. We have obtained stratigraphic and sedi- mentologic criteria for recognizing ancient fan deposits by comparing the Astoria Fan, a large open-ocean fan off the coast of northern Oregon, with the Eocene Butano Sandstone, an ancient continental borderland fan deposit of similar size in the Santa Cruz Mountains, California.
Deep-sea fan deposits consist of channel and interchannel facies. Both facies change significantly downfan and laterally across the fan as a result of decreasing current velocities during each turbidity current and as a result of the lateral migrations of channels through time. Geologic mapping reveals thick-bedded, coarser grained, and lens-shaped channel deposits intermixed with thin-bedded and finer grained interchannel deposits. Sand-shale ratios are high within channels and low within upper fan interchannel and distal fan areas. The coarsest grained and thickest bedded gravels and sands are deposited by channelized sediment gravity flows in the submarine canyons and upper fan valleys. These ungraded, poorly sorted, and massive channel sediments change by midfan to thinner bedded, finer grained, vertically graded, and better sorted turbidite sands that contain sedimentary structures in Bouma sequences. Turbidites in interchannel areas are formed by overbank spilling and consist of thin-bedded fine-grained sands and silts characterized by Bouma cde and de sequences.
The delineation of fan margins and paleogeography is aided by the lateral and downfan changes in thickness, texture, composition, and paleocurrent directions of fan sediments. High contents of terrigenous debris are present in the sand fractions of hemipelagic muds deposited near the continental margin, and this may help delimit the shoreward boundary of the fan; in contrast, gradation to high contents of pelagic material indicates the direction of the seaward edge of the fan. Radially oriented paleocurrent patterns define the fan apex but are typically complex because of lateral overflow out of and away from channels and because of meandering and lateral shifting of fan channels. The outlining of fan geometry and major channels also is assisted by the decrease of the maximum clast size and of thickness of turbidite beds both downfan and laterally from channels.
Variations in morphology, stratigraphy, sedimentary facies patterns, grain-size distribution, sediment composition, and sediment dispersal patterns help identify fans from different geosynclinal settings such as restricted borderland or marginal sea basins, open ocean continental rises, and deep-sea trenches.