PALEOCENE WINDY GAP VOLCANIC MEMBER, MIDDLE PARK FORMATION (NORTH-CENTRAL COLORADO), AND EVIDENCE FOR COMPLEX SEDIMENTATION IN AN AXIAL LARAMIDE FORELAND BASIN

New stratigraphic and geochronological evidence from northern Colorado indicates the timing of Laramide uplift and sedimentation is more complex than previously recognized. West of Granby, Colorado, the Windy Gap Volcanic Member (WGVM; Izett, 1968) is the basal unit of the Laramide foreland-basin fill (Middle Park Formation) in the North Park-Middle Park basin (Colorado Headwaters Basin, CHB, of Cole and others, 2010). Izett (1968) broadly correlated the Middle Park Formation with synorogenic strata of the Denver Basin, based on similar lithologic sequence and sparse pollen and leaf data. Our work shows instead that sedimentation in the CHB began more than 5 m.y. later.

The WGVM is a distinctive cliff-forming unit, 150-230 m thick, consisting of coarse conglomerate containing >50% clasts pyroxene-porphyry trachyandesite and >30% clasts of plagioclase-porphyry volcanic rock. Sedimentary structures and bedding characteristics indicate most of the WGVM was deposited in a lake, and that subsidence and sedimentation rates were high. Ages of various clasts (Ar-Ar; whole-rock, hornblende, feldspar) range between 65.5 Ma and 60.5 Ma, with the youngest date indicating a maximum age for the onset of sedimentation. A dike of pyroxene latite (67.3 Ma; U-Pb zircon) in nearby Cretaceous marine strata and trachybasalt flows (minimum 60 Ma; Ar-Ar) beneath the WGVM several km south of Granby attest to the duration of mafic volcanic activity in this region immediately prior to WGVM deposition. Approximately 2,000 m above the base of the WGVM, fossil leaves, pollen, and volcanic ash in mixed arkosic and volcaniclastic fluvial beds of the Middle Park Formation indicate deposition at 58-57 Ma (I. Miller, pers. commun., 2010), confirming very high sedimentation rates (1,000 m/m.y.).

While the Denver Basin was subsiding and filling (beginning about 68-67 Ma), the area of the CHB was uplifting, eroding more than 1,000 m of Upper Cretaceous marine strata, and elevating an additional 1,000 m above sea level. Once the CHB began to subside following 60.5 Ma, subsidence was rapid overall but episodic, with nearly 3,000 m of basin-fill preserved. The WGVM represents a thick, rapidly deposited lacustrine fan-delta complex adjacent to a mafic volcanic field on the southern margin of the CHB.