Geologic Overview of Jefferson County, Colorado

This report summarizes the results of a study of the mine subsidence hazards and the coal resources of Jefferson County, Colorado. The area covered includes that part of the county generally east of the Hogback, and the subsidence hazard portion of the study is limited to coal and clay mines.

The study area lies in eastern Jefferson County and includes all or parts of the following townships: T:2,3,4,5, and 6S; R.69W., T.2,3,4,5S; R.70W. and T.2S., R.71W. Though the entire area covers approximately 200 square miles it was a strip two to three miles wide along the foothills which received the most attention because nearly all the coal and clay mines are concentrated in this area.

Map of study area. Click this map to see a larger version.

Geology

Bedrock units within the project area range in age from Pennsylvanian to Tertiary. They are mostly of sedimentary origin and were originally deposited in horizontal layers. Since their deposition, the beds along the western edge of the study area have been turned up sharply. The area of upturned edges of sedimentary rocks constitutes the Foothills section of the Front Range. The Hogback, a familiar landmark in this part of the county, is a prominent part of the upturned section. Farther east, in the central and eastern part of the area, the dip of the beds flattens, topographic relief is low, and the terrain becomes typical of the Great Plains.

Geologic Cross Section of Golden.

Only five of the bedrock units shown in the geologic cross section below have been mined for clay and (or) coal. Three of the units, the Lykins and Morrison Formations and the Benton Shale have produced small amounts of clay. The South Platte Formation of the Dakota Group and the Laramie Formation have produced the bulk of the clay in the county, and the Laramie Formation has been the sole producer of coal.

Lykins Formation

The Lykins Formation consists primarily of shale with some thin beds of sandstone and limestone. The shale is brick red in color and is silty or sandy in part. The sandstones are light gray, very fine grained, silty, and occur in beds no greater than one inch thick. The limestones are very light gray to reddish brown, hard, dolomitic and silty with some irregular, wavy (crinkled) bedding. The formation is approximately 450 thick.

Morrison Formation

The Morrison Formation is characterized by its variety of rock types and its variegated colors. It is composed dominantly of siltstones, mudstones and claystones but has some sandstone units and a few thin limestone beds. The siltstones, mudstones and claystones exhibit a variety of colors, ranging from reddish brown to yellowish gray to greenish gray. The mudstones and siltstones are gradational with the claystones. The sandstones are fine to medium grained, light brown to yellowish gray, crossbedded and lenticular. The limestones are gray, dense to crystalline and contain fossil algae and freshwater mollusks. They usually occur as discrete beds which range in thickness from a few inches to two feet. The thickness of the formation is about 300.

Dakota Group

The Dakota Group consists of two units, the Lytle Formation and the overlying South Platte Formation. Both formations are dominantly sandstone units, but both do have significant claystone and siltstone intervals. The sandstones are generally fine to medium grained, massive to crossbedded, and light gray to yellowish brown in color. Individual sandstone beds, though usually massive, are also lenticular and occur as lenses which thicken and thin and overlap each other. The claystones, like the sandstones, are quite lenticular and no one bed can be traced any great distance. The siltstones of the Dakota Group are dark gray in color and vary in composition from clayey to sandy. They are lenticular and have no great lateral extent along the outcrop. All three rock types overlap and compositionally grade into each other. While the beds are individually unmappable they form a composite unit roughly 300 thick which is quite recognizable throughout the project area.

Benton Shale

The Benton Shale in the project area is divided by most workers into three subunits. These are, on the basis of paleontologic evidence, equivalent to rocks which elsewhere are recognized as distinct formations. The lowermost of these units, the Graneros Shale equivalent is about 150 thick and consists of dark-gray clayey shale with a few thin beds of siltstone. The middle unit, the Greenhorn Limestone equivalent, is about 300 thick and consists of black to light-gray clayey shale. It contains a few beds of very light colored bentonite and a few thin, yellowish gray limestone beds. The upper part of the Benton is known as the Carlile Shale equivalent. It is about 50 thick and consists mainly of medium-dark-gray siltstone with a few thin beds of fine grained sandstone.

Laramie Formation

The Laramie Formation is comprised of sandstones, siltstone, claystones, ironstones and coals. The lower part of the unit consists mostly of sandstone and claystone in about equal amounts. The upper part consists mainly of claystone with some thin siltstones. Coal beds and a few thin ironstones are found near the base of the lower part of the Laramie.

The sandstones of the Laramie are medium grained, light gray and massive to thin bedded. They are poorly cemented throughout and are often soft and friable. They may have clay filling the interstices between grains rather than quartz cement.

The Laramie claystones are generally medium to dark gray but may be white or have a reddish tinge. Many of the claystones are silty or sandy and some are lignitic. Subsurface evidence indicates that claystones make up a greater portion of the Laramie as one proceeds eastward from the outcrop. There is also an eastward thickening of the Laramie Formation from about 600 at the outcrop to about 1000 beneath the easternmost part of the county.

According to Van Horn (1976) there are six known coal beds in the Laramie in just the Golden quadrangle. The coals are ranked as subbituminous and range in thickness from a few inches to as much as 14. Their occurrence is limited to the lower 200 of the Laramie. Because the coal beds do not usually form outcrops it is impossible to trace individual coal beds between mines. As a result, coal bed correlation is very poor through out the project area.

A further hindrance to correlation of units within the Laramie is their lenticularity. Individual beds are not continuous over more than a few miles. Many coal mines and some clay mines were confined to a single bed and were forced to close when that bed thinned to an uneconomic thickness.

Structure

Geologic Cross Section of Golden.

The structure of the project area is composed of three major geologic elements; the Front Range uplift, the Golden fault and the Denver basin. The uplift of the Front Range immediately west of the project area has affected the adjacent sedimentary beds on the east. Originally flatlying, these beds have been folded sharply upward and now dip (form an angle with the horizontal) eastward at angles ranging from 30 degrees to nearly vertical. The upturned edges of the more resistant sedimentary beds form north-northwestward-trending hills and ridges found along the western margin of the project area. The most familiar ridge in this Foothills section is the Dakota hogback which extends almost without interruption from the Boulder County line to the South Platte River.

The second large structural feature, the Golden fault, extends from Ralston Dike to Mount Carbon, a distance of about 15 miles. The fault is a westward-dipping, high angle reverse fault which has brought older beds up over younger beds. Its effects are most profound west and southwest of Golden where large parts of the normal stratigraphic sequence have been cut out of the surface section.

Another apparent effect of the movement along the Golden fault has been the steepening of bedrock dips on the east side of the fault. West of the fault trace the rocks commonly dip eastward at angles of 30 to 60 degrees; east of the trace their dips are usually almost vertical to overturned. As a result the clay mines in the Dakota Formation (west of the fault) were worked at moderate to fairly steep angles while the clay and coal mines of the Laramie Formation (east of the fault) were worked along nearly vertical seams.

The Denver basin is a major regional structure which has an east-west width of about 140 miles and, a north-south length of about 350 miles. It reaches its maximum depth of about 13,000 beneath east Denver and the rocks underlying eastern Jefferson County dip eastward toward that point. The steep to nearly vertical dips found near the Dakota hogback and the Golden fault abruptly flatten three to four miles east of the hogback. Beneath the central and eastern parts of the project area the eastward inclination of the beds amounts to only a few degrees.

All of the geologic information contained on this page was taken directly from Coal and Clay Mine Hazard Study and Estimated Unmined Coal Resources, Jefferson County, Colorado by Amuedo and Ivey, 1978