Very Fine to Fine Sandstone (VFS-FS)
This facies consists of angular, well sorted, red, fine to very fine sand with some silt and clay. The sand is 40% quartz, 50% limestone lithic fragments, 3% microcline feldspar, 1% chert, and minor biotite (often highly weathered), chlorite, muscovite, and glauconite. There are few rounded quartz grains (derived from the Cambrian sandstone source rocks). The cement consists mainly of red clay with little calcite or dolomite. There is a slight normal grading in the layer analyzed by thin section and there is a preferential horizontal alignment of grains, especially the micas. This alignment causes a fissility in the rock which can be readily seen in a hand sample. There are often well rounded floating pebbles or cobbles contained within the sand (Figure 11). Layers of this facies commonly have flat bounding surfaces and are on average 2 cm thick, some of which can be traced laterally for 20 meters or more. Less commonly, this facies can be lenticular and up to 25 cm thick.
Figure 11: SQ outcrop, 7-8 m. Facies CC is overlain by GC (white layer) and VFS. Note the floating cobbles in the VFS layer. Sledge hammer is 39 cm long, and up is to the top of the photograph.
The suspended, rounded pebbles and cobbles show that this facies was deposited by an aqueous flow, but the average particle size is very small, so it could not have been deposited by a fast moving flow. Subaerial fluvial deposits are characteristically red from the oxidation of iron minerals especially if the deposits are exposed to the effects of weathering for a long time (Vischer, 1972). This facies is therefore interpreted as a slackwater deposit (Baker et al., 1983). It could have some ferricrete type soil development because of the relative abundance of clays and the iron oxide staining, though the obvious signs of ferricrete formation, such as rootlets, bioturbation (Blatt and Tracy, 1996), desiccation cracks, and carbonate nodules (Miall, 1983) are not observed in the layers. Another possibility is that these layers do not represent soil in situ, but are partly derived from preweathered material or soil from the source area (Harvey, 1984). Further research on the clay mineralogy is needed to determine the extent of the influence of soil forming processes on this facies.
Coarse to Very Coarse Sandstone (CS-VCS)
This facies consists of well sorted, angular, coarse to very coarse sand with a calcite matrix. The sand grains are 55-70% limestone and dolomite, 25-40% quartz, 2% chert, 2% microcline feldspar, and minor hematite, chlorite, biotite, glauconite, clay, and lithic fragments of quartz and limestone. One anomalous bed contains mostly well rounded quartz sand grains. This facies commonly has lenticular bounding surfaces (Figure 12), but only rarely contains faint crossbeds.
Figure 12: NCR outcrop, 6.12 m, lenticular deposit of facies VCS. Pencil is 9 cm long, up is to the top of the photograph.
This is interpreted as a normal stream flow deposit because of the well sorted nature, the presence of mostly more competent grains (quartz and carbonate) as the bulk of the sandstone, and the lenticular geometry. The fact that it is angular and does contain softer minerals such as the micas suggests that the sand has not been transported far from the source rock.
Granule Conglomerate (GC) and Pebble Conglomerate (PC)
The granule conglomerate consists of 90% well rounded and sorted granule sized limestone clasts (-1 to -2 f) with sand and silt fill (Figure 13). Horizontal alignment of granules or rare pebbles is common and less common is subhorizontal alignment of these clasts in foresets (Figure 14). These layers often have lenticular bounding surfaces. If a layer of GC has more than 30% pebbles, it is termed a pebble conglomerate (average clast size 3 cm, ranging from -2 to -6 f ). Because of the similarities between the PC and GC, they are given the same interpretation. They are interpreted as deposits of normal stream flow because of the well sorted nature, the high frequency of bedforms, and lenticular geometry. Layers with foresets are interpreted as a braid bar deposits (Miall, 1977).
Figure 13: NCR outcrop, 2.6 m, facies GCb. Layer has subhorizontal stratification. Pencil is 9 cm long and depositional up is to the top of the photograph.
Figure 14: SQ outcrop, 31 m, facies PCb (outlined in white) and CC. Note the foresets in the pebble conglomerate layer.
Cobble Conglomerate (CC)
This is an unsorted, well rounded conglomerate and has an average clast size of 15 cm (ranging from -6 to -8 f) (Figure 15). It is clast supported, with no bedforms and has a red tinted matrix of 5-25%. The matrix is itself unsorted though it is often be sandy in places. These layers have flat bounding surfaces and bedforms are commonly vague imbrication or cluster bedforms.
Figure 15: SCR outcrop, 11 m, facies CC. Sledge hammer is 39 cm long, depositional up is to the top of the photograph.
The conglomerate is well rounded, clast supported, and contains some imbrication and cluster bedforms, characteristics typical of stream-laid deposits (Nemec and Steel, 1984). It also has large clasts, is unsorted, is often disorganized, and occurs in sheet-like rather than lenticular geometries in the outcrops. These characteristics are more typical of catastrophic flows (Nemec and Steel, 1984). This combination of characteristics leads to the interpretation of CC as a catastrophic flow deposit with varying ratios of water to sediment between beds. This interpretation is similar to paleovalley conglomerates in the foreland of the Alps in France (Gupta, 1998). The vague imbrication and cluster bedforms can be interpreted as resulting from bedload flood conditions (Morrison and Hein, 1987) and the well rounded and clast supported nature of the conglomerate can be attributed to the characteristics of the sediment from which this conglomerate is derived. If little small sediment (sand size and smaller) was available for transport, the catastrophic flow deposit would have little fine grained matrix with which to support the clasts. This finer sediment could have been carried away prior to the catastrophic flow if the sediments entrained in the flow were previously worked by stream flow.
Boulder Conglomerate (BC)
The boulder conglomerate facies is the same as the CC facies except that the average clast is boulder size (average 35 cm, ranging from -8 to -12 f). The interpretation is the same for BC as for CC, but BCb is interpreted as a channel lag deposit because it commonly has lenticular bounding surfaces and a high boulder to matrix ratio.
Cobble Stringer (CST)
The cobble stringer facies is a string of cobbles (average 15 cm) often with well sorted sand in-between (Figure 16). Because it is associated not with one, but with many of the above facies, it is placed in a facies of its own. It is interpreted as a lag deposit from normal stream flow because of the uniformity of the cobble size and the sand matrix in-between the cobbles.
Figure 16: SCR outcrop, 17 m, facies CST. Jacob staff divisions are 10 cm and depositional up is to the top of the photograph.
Angular Conglomerate (AC)
This conglomerate is clast supported with bimodal distribution of grain sizes. The angular, boulder to granule sized clasts (average 10 cm) are all from the same non-ferric limestone source rock as evidenced by staining (Figure 17). The matrix composes 15% of the rock and consists of red clays, small ( < 1 mm ) sized detrital fragments, and diagenic calcite crystals ( also < 1 mm in size ). The matrix looks similar to that made by interstitial pressure solution. There are no sedimentary structures and the bounding surfaces are not visible because this facies is only exposed in a small gully to the side of the SQ outcrop (Figure 18). It is classified as a breccia in the paleovalley wall because of its angularity and the uniform clast lithology. It could be either a sedimentary breccia at the side of the paleovalley or limestone that was weathered in place and altered by pressure solution, or a combination of both.
Figure 17: Stained thin-section of facies AC matrix. Middle clast is 1 mm in diameter. The interstitial material is mostly clay with some detrital and diagenic calcite crystals. Clasts are stained uniformly pink which show that they are composed of non-ferroan limestone (see Appendix 2). The uniformity of color also indicates that they are all from the same source rock.
Figure 18: SQ outcrop, facies AC. Sledge hammer is 39 cm long and the view is looking down on a horizontal surface of the outcrop.
Conglomeratic deposits do not fall into specific categories but exist along a continuum (Figure 20). There is a common thread connecting all of the flow types from debris flow to stream flow (see figure 19 for flow characteristics): an increase in organization, shown by an increase in rounding, sorting, the abundance of sedimentary structures, and an increase in the ratio of water to sediment in the flow. Few deposits are confined to the strict definition of a particular flow and many flows actually change as they progress downstream (Morrison and Hein, 1987) and so occupy a segment on this line rather than a point.
Figure 19 a: Diagnostic characteristics table of deposits from different types of flows. Compiled from summaries and descriptions from the following articles: Nemec and Steel (1984), Wells (1984), Graham (1983), Brayshaw (1984), Friend (1983), Baker (1983), Smith (1987), Harvey (1984), Goodwin and Diffendal (1987), Harms et al. (1982), Costa (1988).
Figure 19 b, c: B. Typical stratigraphic section of a streamflood flow (from Nemec and Steel, 1984). C. Typical stratigraphic section of a debris flow (from Nemec and Steel, 1984).
Figure 19 d, e: Flow type examples (from Nemec and Steel, 1984). D. Braided river deposit, Howden Formation, Norway. Prominent braided stream features include: well rounded clasts, clast supported, sorting of grain sizes, and imbrication (near top of the photo). E. Subaerial debris flow deposit. Debris flow characteristics include: poorly sorted, matrix supported, and angular clasts.
Figure 20: Classification of facies on a continuum of flows going from debris flow to braided stream flow. Scale shows and increase in rounding, sorting, bedforms, and ratio of water to sediment in the flow; facies and flow types are placed into the continuum based on these characteristics.
Each facies as described above has a stand-alone interpretation based solely on the sediments' characteristics. Further classification of the facies by the degree of organization along the continuum yields three distinct groups. Group A (PC, VCS-CS, GC, CST, BCb, and VFS-FS) has a high degree of organization. Group B (CCm, CC, and CCb) is less organized and group C (GCm and PCm) is most disorganized (Figure 20). AC is not classified because of the uncertainty of its origin.
This organizational classification from the sediment characteristics is reflected by the spatial relation of facies in the stratigraphic section (Figure 21). In the sampled section from the Squaw Creek outcrop, from 3-7 m is CC (group B) and between 7-8 m are several group A facies (GC, FS, GC, and CST). This spatial separation of facies is repeated throughout the stratigraphic section of the Crandall Conglomerate (see Appendix 1; SQ 12-15, 28-31.5 and 55-59 m). This spatial relation supports a distinction between groups of organizationally related facies, therefore the organizational groups can be directly associated with specific facies models. The high degree of sorting seen in group A reflects what is seen in deposits of normal braided streams. The less organized group B facies in the middle of the continuum are typical of fluidal flood flow and hyperconcentrated flow. The organization seen in group C is more representative of debris flow deposits. Because the distinction between debris flow, hyperconcentrated flow, and fluidal flood flow are not well understood in the Crandall, these three facies models are grouped together into a single model, the catastrophic flow.
Figure 21: Sample stratigraphic section from the Squaw Creek outcrop. Facies are labeled along with a further classification into stream flow and mass flow categories.