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Ancient Submarine Slope-Fan Systems
Sedimentation Units in Stratified Resedimented Conglomerate, Paleocene Submarine Canyon Fill, Point Lobos, California
Excellent exposures at Point Lobos, California provide a three-dimensional view of coarse-grained submarine canyon fill of Paleocene age. Although most of the conglomerate fill is stratified on a large scale (decimeters to several meters), individual sedimentation units can be clearly delineated only where sandstone interbeds isolate individual beds of conglomerate. Analysis of sandstone-conglomerate interbeds indicates that they commonly occur as couplets.
A typical couplet includes a lower conglomerate member with a sharp basal contact and an upper sandstone member with a poorly defined basal contact. Both members tend to be lenticular, generally less than a meter thick and no more than a few tens of meters in lateral extent. The conglomerate member typically is inversely graded and unstratified, whereas the sandstone member generally is normally graded and commonly shows stratification. Where seen in section parallel to paleotransport direction, the conglomerate coarsens in the down-transport direction until it terminates laterally as an accumulation of large clasts floating in the sandstone member. Clast long-axes that dip in an up-transport direction commonly define imbrication within the conglomerate. In contrast, clasts isolated in the sandstone member (and at the down-transport terminus of the conglomerate tend to lie with long axes normal to paleotransport.
The relation of the sandstone to the matrix of the conglomerate suggests that the couplet represents a single sedimentation unit, in which the pebbles moved as a traction carpet dispersed by intergranular collision. The textural gradations in the conglomerate and the general absence of small pebbles in the associated sandstone indicate nearly all of the gravel in the flow was deposited rapidly by frictional freezing. The associated sand flow had sufficient competence to roll a few of the larger clasts forward from the top and nose of the immobilized gravel bed. The size of the largest clasts (decimeters to meters) in the conglomerate suggests transport on relatively steep slopes. Forests within a few sandstone beds, however, that dip at near angle-of-repose in the general paleotransport direction, imply a very gentle gradient at the site of deposition. Comparison with other resedimented conglomerates indicates that the dominantly inversely graded conglomerate at Point Lobos represents a highly proximal facies.
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