The Crandall Conglomerate crops out in the Clarks Fork of the Yellowstone River Valley in northwestern Wyoming, between the Beartooth Uplift and the Absaroka Mountains (Figure 1). The major tectonic event that has affected this area was the Laramide orogeny (Cretaceous through the early Eocene, 80-50 Ma) (Bown, 1982). As the Laramide orogeny stopped uplifting in the early middle Eocene (~50 Ma), Absaroka volcanism started to the northwest of the study area and continued towards the southeast (Bown, 1982). At the start of volcanism, the Heart Mountain Fault experienced great movement (see Schoenbohm, 1997, Beutner and Craven, 1996 for further description of the fault). The Heart Mountain Fault is a large, low-angle normal fault along which rocks were displaced many tens of kilometers. The Crandall Conglomerate is cut by this fault, and the direction of movement of the hanging wall of the fault is reflected in the displacement of the upper part of the conglomerate section incorporated into the hanging wall about 20 kilometers to the SE (Figure 1).
Figure 1: Outcrops of the Crandall Conglomerate. Outcrops studied are Squaw Creek outcrop (SQ), North Crandall Creek outcrop (NCR), South Crandall Creek outcrop (SCR), and Beam Gulch outcrop (BG). Dotted box is the location of figure 2.
The age of the Crandall can be constrained by considering what could cause the incision of a paleovalley and then subsequent deposition. Incision is caused by many mechanisms: tectonic events, large scale flooding events, climatic change, (French, 1987) and changes in base level (Maizels, 1983). These factors can affect each other, but tectonic activity tends to be the overriding factor, for it can affect all of the others. The conglomerate is younger than 350 ma because the youngest clasts in the conglomerate are Mississippian in age (Pierce, 1973). Only one tectonic episode that affected this area after 350 Ma, the Laramide Orogeny, had enough influence to initiate stream incision and clastic deposition (Ed Beutner, personal communication).
The volcanic rocks exposed in the study area are Absaroka volcanics. However, no volcanic clasts are observed in the conglomerate, therefore the conglomerate must be older than the Absarokas. This, combined with the Laramide orogenic episode, places the age of the Crandall between the start of the Paleocene and the end of the early Eocene (or 66 to 50 Ma). In addition, the conglomerate shows similarity to many "fanglomerates" located throughout Idaho, Montana, and Wyoming. These conglomerates are all related to Laramide tectonic activity and Absaroka volcanism and have been dated to the early-mid Eocene (Fields, et al., 1985).
Past interpretations of the Crandall Conglomerate have been intimately connected with models for Heart Mountain Faulting and explanations of the origin of the Blacktail Fold, a fold-thrust in the limestone underneath the Crandall Conglomerate (Figure 2). The purpose of my study is to understand the Crandall Conglomerate as a sedimentary unit apart from its connection to these structural features. This deposit is also the only one in the Clarks Fork Valley that represents its age in the stratigraphy. Understanding the signature of climatic and tectonic events in the conglomerate gives a better picture of these mountains as they were forming.
Figure 2: Footwall outcrops and the Blacktail Fold-thrust. Note the emplacement of the Crandall outcrops asymmetrically to the NE of the Blacktail Fold-thrust. Rose diagram of clast imbrication shows paleoflow direction to the east (number of measurments=55, circle=13%).
The investigation was limited to the sedimentology and stratigraphy of three outcrops in the footwall and one in the hanging wall of the Heart Mountain Fault (Figure 1). Lab work consisted of thin section analysis of selected samples from these outcrops and paleoflow analysis using cobble imbrication data collected in the field. The footwall outcrops give a clear picture of the conglomerate in situ, though not the total picture, because the hanging wall exposures contain finer sediments overall (more sandstone and gravel) than the footwall exposures (Pierce, 1973). This difference in the stratigraphy could reflect changes in the environment through time.