The Crandall Conglomerate is deposited in paleovalleys up to 450 m wide and at least 160 m deep cut into Paleozoic limestones (Pierce, 1973). This paleovalley geometry can be seen when looking at the outcrop at Squaw Creek from across the Clarks Fork Valley (Figure 3). The paleovalley exposures in the footwall limestones of the Heart Mountain Fault are positioned asymmetrically (slightly to the northeast) over the Blacktail Fold (Figures 2 and 4). Research done on this fold this year by Steve Di Benedetto of Franklin and Marshall College has shown that this fold was created by Laramide age orogenic activity (Di Benedetto, 1998, personal communication).
Figure 3: The Squaw Creek outdrop (Tc) (from Pierce, 1973). The upper boundary of the conglomerate is the Heart Mountain Fault, and it is overlain by volcanics (Tw). Layers surrounding the conglomerate in the footwall are Paleozoic Limestones.
Figure 4: The Blacktail Fold and the Crandall Conglomerate (Tc). The Crandall Conglomerate does not rest directly above the Blacktail Fold, but slightly to the northeast.
The conglomerate is a subaerial fluvial conglomerate. The majority of clasts are rounded and cobble sized (Figure 5). The roundness and the large size points to transport by aqueous flow. There is an conspicuous lack of fine grained sediments (clays and silt) and fossils in the Crandall Conglomerate that one would expect to find in estuarine and marine environments (Nemec and Steel, 1984). The area was being uplifted at the time of deposition; this created an unconformity in neighboring areas in the Clarks Fork Valley while the Crandall was being deposited. This further supports that this fluvial conglomerate is subaerial.
Figure 5: South Crandall Creek (SCR) outcrop, sledge hammer (circled) is 39 cm long. Up is to the top of the photograph.
The conglomerate is composed of about 95% limestone clasts. These clasts were derived from the units through which the paleofluvial system cut, incorporating the basement granitic rock through the Mississippian Madison Limestone (Pierce, 1973) (Figure 6). Sedimentary features are rare and no woody debris or organic matter has been observed in the conglomerate. Clay sediment is sparse throughout the conglomerate in general.
Figure 6: Stratigraphic column of the Clarks Fork of the Yellowstone River Valley bedrocks (after Hague, 1985). Position of the Crandall Conglomerate is shown cutting to the lowest level in the stratigraphy that is observed in the field.
Strat column
Overall the outcrops have a red color which comes from red lithic grains and iron oxide staining in the matrix. The matrix is 80% granules or pebbles of which 95% are limestone. Of the fraction sand size and smaller, 45% is quartz, 50% is limestone, 2% is microcline feldspar and 3% are accessory minerals. Most of the matrix less than granule sized is not well sorted or rounded with the exception of sparse rounded quartz grains coming from Cambrian sandstones and rounded glauconite grains. These glauconite grains are also present in some of the limestone clasts. The remainder of the accessory grains are red clay and small aggregates of quartz, calcite, and clay (Figure 7). The aggregates are deformed at the edges and so appear to have been soft when deposited. They could either be from the shale source units or could be rip up clasts from sediment in the paleovalley at the time of deposition. There is some deformation of these deposits as evidenced by fractured quartz in the matrix.
Figure 7: Thin-section of a red clay aggregate with floating quartz crystals found in the conglomerate matrix. The aggregate is 2 mm across and had deformed edges.
Figure 8: Angular block of limestone in the conglomerate at the base of the NCR exposure (towards the right bottom corner in the picture). Sledge hammer is 39 cm long.
There are large blocks over a meter in diameter in several of the outcrops, none of which are well rounded (Figure 9). These are interpreted as debris fallen down from the side of the valley walls, though no soft sediment deformation from slumping is observed around them (Wells, 1984). Imbrication and foresets used to measure paleocurrent were rare and often poorly developed, so the flow direction is not well constrained. Paleoflow measurements obtained (Figure 2) show a flow direction to the east.
Figure 9 a, b: The most well-developed imbrication examples from the Crandall Conglomerate. A. SCR outcrop, 9m, facies PCb. B. SCR outcrop, 16m, facies CCb.
Facies Descriptions and Interpretations
Each facies of the classification can have a subset of either "m" or "b" (i.e. CCm). The "m" denotes that the particular layer is matrix supported and the "b" denotes that there is a bedform of some kind in the layer. Bedforms differ from facies to facies and are described below where appropriate. Bedforms are rare, but angular clasts and flat bounding surfaces are common in matrix supported layers. These features are also commonly observed in debris flow deposits (Nemec and Steel, 1984); therefore layers with the subclassification of "m" are interpreted as debris flow deposits (Figure 10).
Figure 10: SQ outcrop, 69.5 m, facies GCm. The clasts are pebble size and are suppported in a gravelly sand matrix. Pencil is 9 cm long and up is to the top of the photograph.