STRATIGRAPHIC AND DEPOSITIONAL RELATIONSHIPS, LATE CRETACEOUS AND TERTIARY DEPOSITS, CENTENNIAL, COLORADO: IMPLICATIONS FOR EOCENE PALEOGEOGRAPHY AND ITS CONTROL ON SHALLOW GROUNDWATER AND CONTAMINANT FLOW

Subsurface characterization activities at a site in Centennial, Colorado have provided data to help construct the depositional setting and paleogeography of the Denver Formation, Dawson Formation, and Quaternary alluvium. This complex geologic setting has a significant influence on shallow, unconfined groundwater flow and contaminant transport.

The Late Cretaceous-early Paleocene Denver Formation is characterized by fine-grained silt and clay fluvial deposits. Lenticular sandstones are present locally, encased in fine-grained deposits. In the early Paleocene, regional uplift led to a period of non-deposition and erosion and the exposed Denver Formation was eroded down to a relatively flat surface by late Paleocene time. The overlying Dawson Formation was deposited in a meandering stream environment lateral to the newly uplifted Front Range. Early in Dawson time, streams cut into the upper portions of the exposed Denver Formation as they crossed the site area and lower Dawson fluvial material was deposited. A period of non-deposition and erosion took place throughout the Pliocene. Pleistocene and recent streams have cut into exposed Dawson and Denver Formations and deposited sequences of alluvial material.

Isopach maps of Dawson sandstones indicate that channel sands deposited during early Dawson time were located near present-day drainages. The thicker fluvial sequences correspond with deeper channels cut into the underlying Denver Formation. This indicates that present-day topography began to develop early in Dawson time. Unconfined groundwater flow in the Dawson generally mirrors that in the overlying Quaternary alluvial aquifer (e.g., topographically controlled). Contaminant groundwater chemistry from groundwater monitoring wells supports these conclusions. In addition, recognition of unconformities between the Denver and Dawson Formations, and between the Quaternary alluvium and underlying bedrock units is critical to understanding depositional and stratigraphic relationships relevant to contaminant distribution and flow.