Ancient Continental Slope Secuences And Their Usefullness In Understanding Modern Slope Development
Authors: Cook Harry E. , Chamberlain W.C. 
Published: October 2008

Field and laboratory work show that a north trending continental slope existed in central Nevada during the Late Cambrian and earliest Ordovician. The regional depositional regime changes from a deepwater slope in the west, represented by 150m of pelagic limestone, to a shoal-water shelf 170 km to the east, composed of 600 m of coeval limestone. Slope and shelf environment prograded seaward with time. Seaward of this relatively thin slope sequence, 1,000 to 2,000 m of sediment forms a submarine fan.

Slope development involved both depositional and erosional processes. Up to 50 percent of the slope sequence is composed of slide, slump, debris flow and turbidity flow deposits. These deposits reflect extensive mobilization of semi consolidated to unconsolidated sediments from both slope and shoal water shelf sites to the east. The upper slope is characterized by translational and rotational slides 10 to 20 m thick and 400 m wide and massive boulder bearing debris-flow deposits that occur in channels up to 10 m deep and 300 to 400 m wide. The zones of basal shear, and thin-tapering margins of the slides were transformed into conglomeratic debris and turbidity flows through mechanical shock, strain, and incorporation of water. Mass transport deposits lower on the slope consist of slides less than a meter thick and a few ten of meters wide, and numerous conglomeratic debris and turbidity-flow sediment in channels 1 to 2 m deep and 20 to 50 m wide. The remainder of the section shows no evidence of remobilization and consists of dark, fine-grained, thin bedded, and laminated lime mudstone which forms the in situ slope deposits, This limestone represents pelagic and hemipelagic sedimentation.

Ancient slope sequences provide an excellent opportunity to study the development of a continental slope in space and time, to observe the geometry and stratigraphic position of coarse-grained potential petroleum reservoirs, and to study the contemporaneous changes that occur in slides, slumps, and mass flows during their downslope movement.

It is reasonable to speculate that resedimentation processes may be more significant on modern continental slopes than is presently recognized on the basis of, abundant small-scale mass-transport deposits less than a few meters thick in this and similar ancient slope settings. The smooth undisturbed nature of some modern slopes as seen on continuous seismic reflection profiles may be in part a function of the limited resolving power of the seismic equipment. Slump or flow features less than a quarter of a kilometer long and less than 10 m thick may not be recognized, by present seismic-reflection techniques, especially with commonly used low frequency seismic systems. The implications of this are pertinent to both exploration for petroleum within continental slope sequences and the recognition of geologic hazards on modern slopes:Seismically undetectable coarse-grained debris-flow and turbidity-flow deposits are potential conduits for migrations of hydrocarbons upslope to shoal-water reservoirs or for forming stratigraphic traps on the slope, anda modern slope undergoing numerous small-scale mass movements poses potential geologic hazards.