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Spit System Facies Model - Can This Be Used to Reinterpret Some of the Isolated Shelf Sandstone Ridges in the Cretaceous Western Interior Seaway, Usa?

Johannessen, Peter N.1; Nielsen, Lars H.1
1 Department of Stratigraphy, Geological Survey of Denmark and Greenland, Copenhagen, Denmark.

Spit systems may constitute very large elongated reservoir sandstones. The Holocene-Recent Danish Skagen spit system is 22 km long, 3-7 km wide, and up to 32 m thick. Spit systems are seldom recognized in the pre-Quaternary sedimentary record compared to their common occurrence along present-day coasts and in Quaternary successions. This lack of recognition may partly be due to the lack of widely accepted depositional models describing the facies characteristics of the spit systems and their subaqueous platforms in particular. It is important to understand that far the largest part of a large-scale spit system is deposited below sea level where up to c. 30 m thick wave dominated shallow marine sands can accumulate. The Danish Skagen spit system provides a unique opportunity for establishing a well-constrained depositional model because uplift and erosion have made large windows in outcrops into the preserved facies, while active spit-forming processes can be examined at the young prograding end of the same system. The sedimentary facies of the spit system are grouped into four principal units consisting from below: 1) thick storm sand beds, 2) dune and bar-trough sediments deposited by waves and longshore currents, 3) beach deposits, and 4) peat beds. These four units form a coarsening and shallowing upward sand-dominated succession, up to 32 m thick, which overlies offshore silt and is topped by a diastem overlain by young aeolian dune sand.

The Cretaceous Western Interior Seaway in North America contains numerous isolated shelf sandstone ridges providing a large amount of hydrocarbons. There has been a long debate on how these sandstone ridges were deposited. We propose yet another interpretation, namely large sandy spit systems. The Western Interior Seaway was characterized by strong waves that caused longshore sand drift toward the south and could probably favour the formation of large sandy, southward-prograding spit systems. Aeolian dune sand, peat beds deposited in swales between beach ridges and beach sand which characterise the uppermost part of the Danish Skagen spit system are not recognized in the isolated shelf sandstone ridges in the Cretaceous Western Interior Seaway. These sedimentary units were probably eroded away during the following transgression. Only the submarine sedimentary units are preserved after the transgression. During the continued transgression offshore mudstones were deposited upon the shallow marine spit units.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009