Dawson Formation as Expression of Laramide Tectonics: ABSTRACT

The Dawson formation was named the Dawson arkose in 1915 by Richardson, the type locality being at Dawson Butte, 6 miles southwest of Castle Rock. Because much of the stratigraphic section contains more fine clastics and carbonaceous material than arkose, the change in name seems desirable. The writer has differentiated at least six sedimentary members of the Dawson on the southwest flank of the Denver basin in addition to rhyolite flows near and possibly at the top of the formation. On the southeast flank of the basin, eight members have been recognized. Another member develops near the southern end of the southwest flank and spans the trough area of the present basin in its southern part. At the northern end of the present outcrop area, a considerable interval of coarse clastic sediments changes rather abruptly to gray shale with minor amounts of arkose. The Dawson formation on the southwestern flank of the Denver basin is interpreted as a series of alluvial fans and bajada deposits with some intercalated mud flows, all of which were being spread out by streams from the rising Laramide mountains on the west. These fans were formed in varying areas at differing times as uplift was locally intensified or diminished by the shifting of centers of middle Laramide tectonism. Most of the fans coalesce along strike. The section is replete with unconformities. Sediments just east of the fans are mainly fluviatile, becoming lacustrine and paludal in the eastern parts of the basin. The present northwestern limit of the outcrop area contains mainly gray shale of lacustrine origin. Total thickness is in excess of 2,000 feet, but at no one locality on the west flank of the basin are all of the members present because of the oscillatory nature of the fan-forming processes. The formation thins to the north, southeast, and east from the area of its maximum deposition near Sedalia toward the areas of finer-grained sediments. The age of the Dawson was accepted as Eocene by Richardson. He considered it to be the southward and southeastward equivalent of the Arapahoe and Denver formations. Lavington (in 1942) believed there was an unconformity between the Denver and the Dawson, and that the Dawson overlapped the Denver. The present investigation disclosed typical Dawson unconformably overlying the Denver formation. The Dawson unconformably overlies all formations from the Pierre shale to the Denver formation in the Colorado Springs area. Palynological examinations of cuttings from about 750 feet above the base of the Dawson indicate "late Late Cretaceous" age. Earlier work on fossil leaves from near the top of the formation was reported by Richardson to suggest a Green River Eocene age for the containing strata. The Dawson is in fault contact with the older sediments and with Precambrian rocks along much of the mountain front. In places, the Dawson strata at or near the faults are nearly horizontal while at other localities they are steeply dipping, vertical, or overturned. Folding and faulting are present in the basin to a greater extent than has been heretofore recognized. Some of the folds have dips of as much as 45°. Faults are of both normal and thrust types. Deformation took place before, during, and after the deposition of the Dawson. The Dawson formation is thus a good expression of Laramide tectonics both in its origin and in its present structural expression. End_of_Article - Last_Page 414------------