THE LATE CRETACEOUS AND EARLY PALEOCENE MEGAFLORA OF THE DENVER BASIN: RECONSTRUCTING THE VEGETATION, TOPOGRAPHY, AND CLIMATE OF AN EVOLVING SYNOROGENIC LANDSCAPE

The synorogenic strata and enclosed fossil flora of the Denver Basin provide an opportunity to study the evolution of a forested landscape over a 13 million year span. The Western Interior Seaway retreated from Colorado shortly after the deposition of the fossils of the Baculites clinolobatus Zone (69.57 Ma). Coastal deposits of the Laramie Formation contain fossil plants that must have grown near sea level. Overlying strata known as the Arapahoe Conglomerate or the basal beds of D1 synorogenic strata contain crystalline clasts indicating that the nearby Colorado Front Range had been unroofed to its Precambrian basement. The 6 km of structural relief associated with the initiation of the Laramide orogeny must have caused some degree of increased regional topography and large amounts of local relief. Timelines defined by the K-T boundary (65.51 Ma) and the base (65.84 Ma) and top (65.24 Ma) of magnetic polarity subchron C29R are used to define a isochron-bounded body of rock stretching from the conglomerate-rich proximal basin margin to the coal-rich distal basin margin. This pattern is complicated by the extinctions associated with the K-T boundary bolide impact. Also, Cretaceous floras are less well-sampled than Paleocene ones. Palynological, paleomagnetic, radiometric and mammalian data show that only the first 1.5 million years of the Paleocene are represented by strata in the Denver Basin. The thickness of these early Paleocene strata ranges from 285m at the Castle Pines well drilled near the mountain front to 160m at a well drilled near the basin center in Kiowa. Thus the flora of this tapering wedge of strata records the recovery from the K-T boundary and the development of basinal topography within a well-constrained spatiotemporal framework. Floras of the base of the wedge are very low in diversity and vary compositionally downslope. Floras of the upper part of the wedge on the mountainous margin of the basin are stunningly diverse (more than 100 species at the Castle Rock quarry) and preserve the distinct physiognomic signature of modern angiosperm-dominated tropical rainforests: large leaf size, abundant drip tips, and a high percentage of entire margins. These floras appear to have been growing on the wet and windward side of a mountain range with enough elevation and relief to stimulate high levels of precipitation.