Mapping Of Small Scale (Outcrop-Size) Seqimentological Features On Modern Submarine Fans
Authors: Normark William R. , Hess Gordon R. , Spiess F.N. 
Published: October 2008

The deep-tow instrument package or the Marine Physical Laboratory of Scripps Institution of Oceanography, equipped for high-resolution reflection profiling, echo-sounding, side scanning sonar and bottom photography, was deployed on six cruises between 1967 and 1976 to study surface morphology and structure of submarine fans off the west coast of North America. The close proximity of the acoustic and photographic sensors on the deep tow to the sea floor allows detection or sedimentological features not resolvable by conventional surface-ship techniques and too large to be seen in bottom photographs, especially those with negative relief-like channels and depressions. Relief of such features, from 10 m to 1 km across is defined as mesotopographic. Mesotopographic relief common in modern fans closely matches the scale range of sediment logical features seen in good outcrops by field geologists. Comparison of deep-tow studies and field observations can provide better facies distinctions in deep-sea turbidite sediments; the detailed vertical and lateral successions described from ancient rocks, however, must be carefully correlated using the knowledge of morphology and surface sediment distribution obtained from study of modern fans.

INTRODUCTION

The principle of uniformitarianism cannot be applied rigorously in the study of deposits of turbidity currents because the processes or deposition (during a turbidity current event) have not been observed directly in the deep sea. Modern turbidite depositional processes are assumed to be identical to those that left ancient turbidite sequences, and the comparison of modern turbidite basins and turbidites from the rock record have resulted in greatly improved facies distinctions for turbidite sequences.8,9,12,14 This comparison of modern and ancient turbidites is constrained by several problems including a severe limitation on the number of morphologic and sedimentologic features readily observed in both settings. The size (scale) range of features that a marine geologist might observe "through" several thousand meters of water commonly has little overlap with the scale range for turbidite outcrops.

For the field geologist, good exposures of ancient turbidites in outcrop might extend a few meters to more than several hundred meters across. An exposure of this size, common to road cuts and stream beds, is good for observing details or sedimentary structures and bedding sequences, but places a definite upper limit on the size or morphologic features than can be recognized. Occasionally, the land geologist might have an opportunity .to "step back" and observe sedimentary relations on a large scale, say across several kilometers where a sequence is exposed in arid mountainous regions or extensive coastal cliffs. Regardless or the scale of outcrops, it is unusual for the field geologist to determine precisely the geometric and time-stratigraphic relations between the accessible outcrops. The nature of coeval depositional surfaces in ancient turbidites is difficult to determine. The field geologist does have distinct advantages in being able to obtain well-located hand specimens for petrologic and sedimentologic examination and to correlate visually these samples with other units in the exposures.