FACIES DISTRIBUTION OF THE COALMONT FORMATION, NORTHWESTERN NORTH PARK, COLORADO, USA

The Paleocene to Eocene Coalmont Formation, in the North Park portion of the Colorado Headwaters basin of north-central Colorado, preserves a diverse facies history. We used 1:24,000 scale geologic mapping and 1:10,000 scale facies mapping to document the fluvial, lacustrine and lacustrine-deltaic facies relationships within the Coalmont Formation in the northwestern part of the basin. Stratigraphic sections were measured in an effort to define these facies. Small scale photo mapping highlights delta front - delta top - fluvial transitions. Excellent exposures of gently dipping strata allow three-dimensional analysis. Lobe switching events between deltaic facies and within individual delta systems were also demonstrated. Paleocurrent data were analyzed to determine sources of the synorogenic, arkosic sand of the Coalmont Formation. Strategic petrologic sampling further defines local provenance for the Coalmont Formation. Palynological data were reviewed and expanded with new collections in an effort to constrain age, and any paraconformable relationships.

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. The source for these lacustrine deposits included two different river systems based on paleocurrent data. There existed both east and southeast trending river systems that transported sediment into the northwestern part of the basin from the surrounding uplifts. 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 the lake from west and northwest.