ANCIENT BACKWATERS: SLOPE MAGNITUDE AND ITS CONTROL ON DELTAIC FACIES PARTITIONING IN ANCIENT DELTAIC SYSTEMS

A common observation in many ancient clastic wedges is that shoreline deposits are commonly significantly finer than associated fluvial deposits. Abrupt downstream fining in rivers, and increase in tidal or marine influence, may mark the backwater limit, which is controlled primarily by slope, tidal range, and river discharge. In Modern low slope systems (i.e. S < 0.0001), such as the Mississippi and Amazon, backwaters may reach many hundreds of kilometers upstream, whereas they may be on the order of a few tens of kilometers in steeper gradient systems.

Ancient deltaic systems of the Cretaceous interior seaway of North America, including the micro-tidal Turonian Ferron Sandstone Member in Utah, show coarse-grained pebbly-sandstone bedload river deposits that feed shorelines that are medium-to fine-grained sandstones. Cross-sections of channels, allow estimates of depth and area. Grain size and bedforms can be used to estimate formative channel discharge (Qw <1500 cumecs). Within the clastic wedge, the pebble-to sand transition typically lies several tens of kilometers from co-eval shoreline deposits. Slope estimates can be made based on onlap distances of associated coastal prisms, as expressed in stratigraphic cross sections. For the Ferron, slopes > 0.001, are an order-of-magnitude steeper than for the low-gradient continental scale systems, like the Mississippi. This explains the position of the transition from pebble- to sand in the fluvial systems, at tens versus hundreds of kilometers from the shoreline, as well as the lack of pebbles in co-eval shorelines. Estimation of slope and discharge thus can be made in ancient clastic systems and allow prediction of the partitioning of coarse versus fine-grained facies at choke-points, as defined by the backwater limit.