Sedimentology and Stratigraphy of the Coalmont Formation, Northwestern, North Park, Colorado, U.S.A.
AuthorPeterson, Christopher D.
AdvisorTrexler, James H.
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The Paleocene to Eocene Coalmont Formation, in the North Park portion of the Colorado Headwaters basin of north-central Colorado, preserves synorogenic strata from the Laramide orogeny. The Coalmont Formation consists of a variety of fluvial and lacustrine sedimentary facies that have not been documented in depth. Using geologic mapping, measured stratigraphic sections, outcrop scale sedimentology, paleocurrent analysis, sedimentary petrography and fluvial-lacustrine architectural element analysis, this study documents a wide array of sedimentologic and stratigraphic data for the Coalmont Formation. 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 Coalmont Formation can be characterized as sandy braided systems in which sedimentation on linguoid 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 Coalmont Formation records a three-stage evolution of the North Park basin during Paleocene-Eocene time. The facies distribution records the evolution from broad fluvial plains to lacustrine deltas and basinal facies and back into broad fluvial plains. Within the upper portion of the Coalmont Formation, at least two periods of progradation are observed when the deltas and their trunk rivers were able to build well out into the lake basin. Deltaic facies contain well-developed cross-bedded foresets in addition to topset geometries commonly transitioning vertically into fluvial facies and/or paleosols. This study confirms details of the history of a subsiding Paleocene-Eocene lacustrine basin having a minimum persistent water depth of several meters and containing several prograding Gilbert-style deltas building into a lake from west and northwest.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.