Richard N. Hiscott

Carbonate Debris Flows, Cow Head Group, Western Newfoundland

ABSTRACT The Cow Head Group is an allochthonous Cambro-Ordovician sequence of deep-water limestone conglomerates, mudstones to grainstones, minor quartz sandstones, siltstones and shales. Platy-clast conglomerates and boulder-rich megaconglomerates as much as 100 m thick exhibit features similar to those described for subaerial debris flows; that is, flat, sharp bases with only minor evidence for erosion, poorly developed clast fabric with tilting of platy clasts at right angles to the flow direction, boulders floating within flows or projecting above the tops of flows, irregular flow tops indicating primary relief above the sea floor, and tapered flow margins (snouts). Snout geometry and the extent of boulder projection both allow calculation of debris static strength. These results, combined with measured flow thickness, permit calculation of minimum and maximum paleoslope gradients. Original debris static strengths were in the range of 103 to 105 dynes/cm2, similar to the observed range in subaerial flows. Calculations yield bottom gradients of l-l 8°, but independent field data suggest that the paleoslope dip was on the order of only a few degrees. This constraint suggests that debris strength was substantially reduced during flow, probably by excess pore-fluid pressures, thus permitting flow on very gentle slopes. The paleoslope orientations used in this study are substantially different from those previously published by Hubert et at. (1977).

Fabric of Coarse Deep-water Sandstones Tourelle Formation, Quebec, Canada

ABSTRACT The Tourelle Formation (Ordovician) consists of shales and sandstones of several different facies types deposited in the midfan region of a submarine fan complex. Dimensional orientation of quartz grains was measured on 93 core-samples taken from ten unusually thick, coarse sandstone layers, to investigate the mechanism of deposition of such layers. Samples were taken both vertically within a layer and horizontally along the strike (for distances up to 570 m) to study local variation in both grain orientation and imbrication. The layers studied are of two facies types, which also differ in fabric. Both types generally show size grading, from about 1 mm at the base to about 0.2-0.5 mm near the top. Layers showing internal stratification have strong preferred orientation that is consistent both vertically and horizontally within a layer, with long axes of grains parallel to flow and imbricate upcurrent, commonly at angles greater than 20°. In some layers orientation is consistently rotated from sole marks. Within individual (reverse graded, cm-thick) stratification bands, vector magnitude is strongest in the finer part of the band. Layers lacking internal stratification have strongest orientation at the base and top. Some basal samples are bimodal. The middle of the layer is characterized by isotropic or bimodal fabrics. Grain orientation and imbrication are generally highly variable within each layer, although one structureless layer displays consistent orientation and upcurrent imbrication. Stratified and most structureless layers were deposited from turbulent suspensions (turbidity currents): a few massive layers were probably deposited from sandy submarine debris flows. Fabric inconsistency, bimodality or isotropism appears to be related mainly to very rapid deposition from suspension or to deposition from sandy debris flow.

Depositional Framework of Sandy Mid-Fan Complexes of Tourelle Formation, Ordovician, Quebec

The Lower Ordovician Tourelle Formation was deposited on a series of small coalescing submarine fans along the eastern margin of an unstable fore-deep trough during early development of the Taconic orogeny. The following features suggest deposition within the mid-fan area: (1) a high proportion of coarse, massive sandstone layers with less amounts of shale, siltstone, and classic turbidites; (2) the presence of deep sand-filled channels; (3) association with olistostromes; (4) a pronounced segregation of coarse sandstones into amalgamated packets separated by thick muddy horizons; and (5) the presence of both thinning- and thickening-upward sequences. Sand packets are characteristic of the unchanneled mid-fan lobes. Some sandstone packets have thinning- or thickening-upward trends in layer thickness. Most, however, show no systematic layer-thickness variation. Each sandstone packet represents deposition associated with development and basinward progradation of a single mid-fan lobe complex. Lobe abandonment, caused by avulsion upfan, results in accumulation of a mud blanket on top of coarse facies of the active progradational phase. At a later time, the same part of the fan may be reoccupied, allowing deposition of yet another sand packet. Sequences deposited in mid-fan channels differ from lobe sequences in that they are coarser, contain deeper channels, lack thick mud horizons, and contain a higher percentage of asymmetric cycles which thin upward. The present submarine fan model places emphasis on thickening- or thinning-upward cycles for environmental interpretation. These asymmetric cycles are, however, uncommon in the Tourelle Formation. If flow size is controlled by a combination of external factors such as rate of sediment supply, frequency of major storms or earthquakes to initiate submarine mass flows, etc, then environmental controls on layer thickness will be masked, and recognizable asymmetric cycles will not be common. External control of layer thickness trends will not, however, affect the observed stratigraphic segregation of coarse and fine facies which characterizes the depositional lobes. This development of alternating amalgamated sandstone packets and mud blankets may be a more useful criterion for recognition of ancient mid-fan lobe sequences.