Magnetostratigraphy of Upper Cretaceous (Maastrichtian) to lower Eocene strata of the Denver Basin, Colorado

Abstract

The Denver Basin is a Laramide foreland basin that filled with synorogenic sediment shed from the rising Rocky Mountains from the end of the Cretaceous through the Eocene. This sedimentary sequence contains a rich and diverse biota that is difficult to correlate because of the low relief and poor exposures characteristic of this region. This study has correlated the stratigraphic sequence and fossil localities using a combination of three techniques. Magnetostratigraphy has proven to be an effective way to date these rocks as they contain a measurable and interpretable reversal sequence that can be correlated to the latest Cretaceous through Tertiary geomagnetic polarity time scale (GPTS). The palynostratigraphy of the region is well known and the rocks contain multiple levels that yield palynomorphs. Volcanic ashes found in both the Cretaceous and Tertiary units can be dated using the 40Ar/39 Ar isotopic dating method. Each technique has its advantages and disadvantages, but in combination, these three chrono- and biostratigraphic methods have the potential to date with a high level of precision virtually every fossil locality sampled in the Denver Basin. To effectively date the exposures from across the entire basin, a reference or benchmark section had to be established against which the biostratigraphic zonation could be directly correlated to the chronology. Due to the paucity of long surface outcrops, drilled wells were the only way to obtain a continuous rock sequence from which a reference section could be constructed. Two cores, one drilled at Castle Pines on the western margin of the basin, and another at Kiowa in the central part of the basin, provided a continuous rock sequence from the top of the Maastrichtian Pierre Shale to the Eocene rocks of the D2 synorogenic sequence. The magnetostratigraphic study of these cores established a reversal sequence that could be correlated to the GPTS ranging from polarity chron 31 through to chron 24. The palynostratigraphy yielded a zonation ranging from the Aquilapollenites striatus Interval Zone through to the early Eocene, and accurately placed the Cretaceous-Tertiary (K-T) boundary in each core. Isotopic ages were obtained from the Maastrichtian, early Paleocene, and early Eocene parts of the section, and allow us to independently confirm the calibration of the units to the time scale. With this chronostratigraphic framework in place, the individual fossil-bearing localities and surface sections from across the entire basin can be correlated and dated to a precision that is comparable to the calibrating isotopic ages. The measured sedimentation rates vary across this asymmetric basin, with higher rates in the western, proximal part of the basin, and a pronounced increase in sedimentation rate across the K-T boundary. Two separate packages of strata, separated by a regional unconformity between the D1 and D2 synorogenic sequences, were dated. The Maastrichtian through Paleocene sequence that encompasses the Pierre Shale, Fox Hills Sandstone, Laramie Formation, and D1 strata dates from about 69 to 64 Ma. The overlying D2 synorogenic strata are poorly constrained and date from about 54 Ma. The reversal pattern of the D2 sequence varies across the basin, which indicates that sedimentary hiatuses, probably caused by tectonically quiet intervals along the mountain front, were followed by differential subsidence across the basin as sedimentation resumed at different times across the 100 km (60 mi) breadth of the basin.