CROSS SECTIONS OF THE NORTH PARK-MIDDLE PARK AREA, COLORADO ILLUSTRATE REGIONAL BASIN HISTORY

The North Park – Middle Park Basin in northern Colorado formed as an intermontaine basin in the latest Cretaceous to early Paleocene during the Laramide Orogeny. Several ~67-65 Ma lava flows and a ~61Ma volcaniclastic conglomerate (Windy Gap Volcanic Member of the Middle Park Formation) are the oldest preserved Laramide synorogenic deposits associated with this basin. These basal deposits are only present in the southern part of the study area, suggesting the basin did not fill in synchronously. The youngest deposits of the basin-fill succession are only preserved in the northern part of the basin and are estimated to be as young as early to middle Eocene. Throughout the basin there is surface and subsurface evidence for deformation before, during, and after deposition of the Paleocene – Eocene synorogenic basin fill, illustrated in the cross sections. A distinct pre-depositional phase of folding, faulting and tilting affects the underlying strata, but not overlying basin fill. Angular unconformities and large stratal offsets exist locally below the base of the basin-fill sediments. Syn- and post-depositional folding and faulting affect the Laramide strata, however progessive unconformities are hard to demonstrate within the sediments. Youngest (mostly extensional) folding and faulting involves Oligocene to Miocene volcanic and sedimentary rocks of the North Park Formation in the north and the Troublesome Formation in the south.