Architecture of Interfingering Fluvial and Lacustrine Facies of Gilbert Delta Systems in the Coalmont Formation, North Park Colorado

Peterson, Christopher D. and Trexler, James H. Jr.
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Gilbert-type lacustrine deltas have previously been recognized in the Paleocene-Eocene Coalmont Formaton of North Park Colorado, but until now researchers have not applied a fluvial architectural analysis to them and their contributing fluvial systems. Using the architectural element and lithofacies ideas of Miall (1985) the fluvial systems of the Paleocene to Eocene Coalmont Formation can be characterized as sandy braided systems in which sedimentation on lingouid and longitudinal bars was common. The fluvial systems vary from coarse gravel dominant, braided facies through more fine grained meandering streams and tributaries. Building on fluvial architectural element analysis, the lacustrine deltas of the Coalmont Formation have many of the fluvial elements defined by Miall which can be used in analysis of the fluvial lacustrine transition. Delta sedimentation was dominated by sediment gravity flows, laminated sand sheets, and abandoned channel elements of Miall (1985). We have defined a new element of inter-lobe and distal lobe fine grained sediments to account for lobe avulsion related fine grained lacustrine sedimentation atop and stratigraphically between lobes of these Gilbert-type deltas.

The North Park portion of the larger Colorado Headwaters Basin provides an excellent opportunity to document Laramide synorogenic sedimentation into an axial lacustrine basin. The Colorado Headwaters Basin was an open freshwater lacustrine setting during deposition of the immature arkosic sediment of the Coalmont Formation, evidenced by the lack of lacustrine carbonate rocks and evaporites within the stratal succession. The Coalmont Formation is dominated by fluvial and deltaic sedimentation and the predominance of fluvial facies provides an excellent opportunity to study and document the fluvial-lacustrine transition zone.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013