DMNS-Denver Basin Project

Stratigraphic Research: Directions and Progress
by Bob Raynolds

Stratigraphy is the branch of geology that deals with the analysis of layered sedimentary rocks. We use stratigraphy to help us decipher the paragraphs, pages, and chapters that make up the rock record. This record is like a book, which can reveal the history of the sedimentary rocks that have accumulated layer by layer through time.

Our goal is to read the rock record clearly enough to permit us to reconstruct the history of deposition and to re-create the succession of ancient landscapes that once existed in the Denver Basin. This insight will help us develop a capability to describe the nature of the sediments (and their potential fossil and groundwater content) in areas where there are inadequate data.

Our collective efforts to unravel the history of deposition of the sediments in the Denver Basin can be divided into three linked topical areas: chronology (timing of rock accumulation), paleoenvironments (understanding ancient climates and ecosystems), and paleogeomorphology (understanding ancient landscapes).

The stratigraphic research involves two principal approaches, one macroscopic, one microscopic.

On a macroscopic scale, I am striving to work out the large-scale geometry of the layers of sedimentary rock and their significant boundaries. These sedimentary rocks were deposited by a series of river systems feeding from the uplifting Front Range of the Rocky Mountains. This analysis is accomplished by building correlated databases showing the thickness and distribution of the various layers of rock. By examining thickness relations, sizes of pebbles and clasts, as well as ancient current direction indicators (paleocurrent indicators such as cross beds) I can make educated guesses (working models or hypotheses) describing where the sediments came from and what the basin looked like as the rocks accumulated. By looking at the composition of the sediments (sand versus shale, composition of sands) I can test and further refine these hypotheses. My present analysis, which builds strongly on earlier work by U.S. Geological Survey geologist Paul Soister, indicates that there are two principal packages of synorogenic (deposited concurrently with mountain building) sediment in the Denver Basin separated by an ancient weathering surface or paleosol. The first package, which I term D1 (D is for Denver), was deposited at the very end of the Cretaceous and into the earliest Tertiary (about 69 to 64 million years ago). The second package (D2) was deposited in the Eocene (about 55 to 50 million years ago). Thickness patterns suggest that the first package was derived from the Front Range and deposited in an asymmetrical basin, which developed parallel to this growing mountain belt. The second package thickens to the southwest, suggesting that it may have had a source near Colorado Springs, perhaps from the Pikes Peak massif.

On a more detailed level, I am taking the basin apart sand grain by sand grain. With the help of two interns from Kent Denver School, Nicole Russo and Andrea Roady, I have taken a series of samples of sandstone from the Kiowa core and disaggregated the sand grains and analyzed them with both optical and SEM microscopes. A fascinating story is revealed by the character and composition of the individual sand grains. Most of the grains are quartz, feldspar and mica; materials readily available in the Front Range area. On close inspection, some of the quartz grains are extremely well-rounded, almost spherical. The grains must have traveled very long distances or existed in high-energy settings like beaches to have become so round. I believe that they are recycled out of earlier sediments (like the Dakota Sandstone), which would have been present and breached to erosion as the mountains uplifted. Other, more rare, quartz grains are beautiful double pyramids showing perfect crystal forms with only the slightest abrasion. I interpret these to have come from an igneous source (the crystals having formed by slow cooling in a magma chamber) that must not be very far away—perhaps near Berthoud Pass.

Each sand grain has a story to tell, and I am studying the vertical succession of these stories to help read the history of the uplift of the Rockies. Integrated with the larger-scale picture, I should be able to better characterize sediment supply and dispersal patterns through time and thus to arrive at a high-resolution picture of deposition in the Denver Basin.