Stratigraphy of the Upper Bridger Formation (Twin Buttes and Turtle Bluff members)

   




Stratigraphy is the science of rock layers, or strata, and it involves the study of all aspects of these strata, including interpretations regarding their origin and geologic history. Of particular importance is the arrangement of rock strata in terms of geographic location (horizontal) and chronologic (vertical) order of sequence.

 


Looking east at horizontally bedded rocks of the upper Bridger D (uppermost subdivision of Twin Buttes Member) and overlying Bridger E (Turtle Bluffs Member) on the southwest flank of Cedar Mountain, Uinta County, Wyoming. The red beds constitute the Turtle Bluff Member.



Reconstruction of the Bridgerian carnivorous mammal Mesonyx obtusidens scavenging a Uintathere skull (as depicted by well known paleo-artist Charles Knight).

Stratigraphers use rock layers to subdivide rock formations, because a single layer of rock that can be traced and mapped over a significant geographic distance represents a relatively synchronous time-horizon. Thus, although there are exceptions, a rock layer deposited in one area of a depositional basin was deposited at about the same time as that same layer many miles away. Because volcanic ash-fall layers are deposited within weeks or months, they represent the most time-synchronous rock units, and in terms of geologic time, are instantaneously deposited.



Although a geologic “formation” is the fundamental lithostratigraphic unit, formations may be further subdivided. For example, formations are commonly subdivided into subordinate units called “members.” These are sometimes divided into submembers, or even finer subdivisions. For example, the Bridger Formation is comprised of three members which are separated stratigraphically by limestone beds. These are the Blacks Fork, Twin Buttes, and Turtle Bluff, and even finer subdivisions are discussed below. The smallest subdivision would be an individual bed of rock, such as a “marker bed.” Marker beds, which are so-called because they are laterally continuous and thus mappable over large distances, are used to define the boundaries between overlying and underlying rocks, and to correlate rock sequences across often large geographic areas.

 


Generalized stratigraphic section of the upper Bridger Formation in the vicinity of Hickey Monutain, Sage Creek Mountain, and westernmost Cedar Mountain, Uinta County, Wyoming (from Murphey and Evanoff, 2006).


Looking south from the south end of Black Mountain across the "saddle" to the north side of Twin Buttes, Sweetwater County, Wyoming. The Lonetree limestone, which is the base of the lower Bridger D subdivision, is the thin light gray bed on the slope of Twin Buttes in the approximate center of the photograph. Clasts of weathered Oligocene-age Bishop Conglomerate litter the ground surface in the foreground.

Much like a layer cake, the Bridger Formation is comprised of rock strata that are almost horizontal. Some of these strata are individual layers of rock that can be traced for more than 50 kilometers. These are mostly thin limestone beds and ash-fall tuffs (less than 1.5 m thick) which are typically lighter in color than the dominant green and brown sands, muds and clays with which they are interbedded. The limestone beds are also generally more resistant than other types of Bridger rock, and so they commonly form ledges that support widespread mesas and benches. The most widespread and well-exposed of these rock layers have been used as stratigraphic marker beds to subdivide the Bridger Formation.

 




The first stratigraphic subdivisions of the Bridger Formation were proposed by W.D. Matthew in 1909, who subdivided the unit into five subunits: A-E, from lowest to highest (oldest to youngest). As discussed previously, Matthew’s stratigraphic work was a pioneering effort which resulted in the first scientifically documented collections of fossils from the Bridger. The marker beds he used as boundaries between his stratigraphic subdivisions were all limestones. He named them the Cottonwood, Sage Creek, Burnt Fork, Lonetree, and upper white layers, from lowest to highest. Little additional detailed stratigraphic or geologic work was done in the Bridger Formation until the 1990’s.

 

 



W.D. Matthew's classic geologic map of the Bridger Basin (1909). The thin dashed line in the lower half of the map is the line of section illustrated in the correlation diagram below.



W.D. Matthew's (1909) correlation diagram of the Bridger Formation in the southern Green River Basin, Uinta and Sweetwater counties, Wyoming (from generally west [left] to east [right]). Matthew's subdivisions A-E are shown.

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