BASIN EVOLUTION AND PALEOGEOGRAPHY OF AN AXIAL LARAMIDE BASIN, NORTH PARK-MIDDLE PARK, COLORADO

The North Park-Middle Park basin contains more than 2.5 km of clastic strata deposited during the Laramide Orogeny. It is currently surrounded by basement-cored mountain ranges, but we show that these uplifts largely postdate basin sedimentation. Well log data, field observations, palynology, paleobotany geochronology all contribute to our interpretation of the complicated fill of this basin.

Prior to basin subsidence in the middle Paleocene, broad uplift and local deformation with large fault offsets occurred in the area. The southern part of the basin started to collect sediments first, predominantly volcaniclastic in nature. Clasts within the deposit are mostly rounded and span a large range of ages of which the youngest are dated 60.5 Ma. Chemical analyses on igneous porphyry clasts within this lower unit but also in the younger synorogenic deposits in the basin indicate source areas as far as 120 km south of the basin. A phase of extremely rapid subsidence followed with sediment accumulation rates of over 750 m/m.y. Predominantly fluvial to paludal and sometimes lacustrine sediments of alternating volcanic and basement origin filled the basin at this time. Fossil leaves are common in the section and leaf margin analysis by the Denver Museum of Nature and Science suggests that the region was significantly uplifted relative to the nearby Denver Basin.

Along the northern and western margins of the basin synorogenic deposition started 1-2 m.y. later than in the south, based on palynology. Basal deposits in these areas mostly consist of relatively finer grained fluvial and paludal sediments suggesting that there were no actively uplifting regions nearby. However, angular unconformities within the synorogenic strata in the western part of the basin demonstrate deformation during deposition in the late Paleocene or early Eocene, after which time the basin filled with fluvial, deltaic and lacustrine deposits.