Origin of Large-Scale Inclined-Bed Sets in Ancient Tide-Influenced Delta Deposits

Brian J. Willis
Texas A&M University, College Station, TX

It is difficult to interpret the origin of large-scale tidal inclined-bed sets (meters to a few 10s m thick, and several km in length and width) within ancient tide-influenced river delta deposits because there is a lack of adequate facies models and most ancient tidal deposits have, until recently, been interpreted as estuarine. Beds within sets generally coarsen and thicken upward, decrease in dip from a few degrees (rarely a few tens of degrees) within upper parts of sets until they are tangential to basal erosion surfaces, and contain smaller-scale internal dune and ripple-scale cross sets that show influence of reversing currents. Although such bedsets are commonly interpreted to be deposits of elongate bars or “ridges” (similar to those observed today at distal edges of many tide-influenced deltaic shorelines), in many cases they may record the grow and migration of 1) tidal channel bars, 2) bars formed under less channelized flows on subtidal delta tops, 3) “mouth bar” like bodies that prograded onto a delta front, or 4) prodelta dunes (and potentially other types of flow-transverse elongated bedforms). Distinguishing deposits of these different depositional settings is difficult without independent estimates of water depth. There is a lack of studies that report: 1) details of dip and strike facies variations within bedsets, 2) geometry of bedset elongation relative to ancient shorelines, and 3) variations in of different scales of cross stratification that can be used to discriminate bedforms with different shape, grain sorting patterns, and amounts of lateral vs. along flow accretion.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005