The Sedimentology of Silt Turbidites from the Gulf of Alaska

Abstract

Deep-water silt beds apparently deposited by turbidity currents are an important lithology in the Gulf of Alaska. Graded beds of moderately sorted silt of modal size 4 to 50, often well laminated, may correspond to the Bouma B and C divisions in sand turbidites. Thick, poorly sorted, unlaminated silts with little grading may be equivalent to the A division. Poorly sorted silts are also found in the D division, perhaps as a result of rapid cohesive clay deposition. Some thin, well sorted, 60 silts are found, which may be derived from turbidity currents formed by direct flow of sediment-laden, glacial outwash streams into the sea. Thin silt laminae are common, both singly and in groups. At least some were deposited by turbidity currents. Turbidite and hemipelagic mud can sometimes be distinguished by the distribution of medium sand and coarser sediment of glacial erratic origin. In other cases, a hard fissile mud (?pelagic) overlies softer, darker, often coarser mud (?turbidite) which in turn overlies turbidite silt. Mud of turbidite origin appears to be an important sediment component. INTRODUCTION This paper describes deep-water silt beds found in the Gulf of Alaska on Leg 18 of the Deep Sea Drilling Project. Most are of Pleistocene age, but some date back to the Miocene. Figure 1 shows the location of DSDP Sites 177 to 181, from which material was obtained. Detailed descriptions of the sites are to be found else where in this volume. At these sites, beds are predominately of silt interbedded with mud. Sand beds are very rare. The sediment in the silt beds has a modal size of medium or coarse silt, and often contains 5 to 15 percent very fine sand. The beds are usually sharp based and graded; the modal size of sediment at the base of a few beds is within the very fine sand range. The silt beds studies are believed to be turbidites, rather than the deposits of bottom currents, for the following reasons: 1) The overall rates of deposition are very high (up to 1800 m/my). 2) Derived shallow-water microfossils are common. 3) There are no clearly winnowed deposits, despite the abundance of poorly sorted glacial mud with erratic pebbles, granules and sand, which would produce a distinct deposit if winnowed. Neither is biogenous debris an important component of the silt beds, despite its abundance in some muds. 4) Many of the silt beds are graded, and have sharp bases. 5) There is evidence of rapid deposition and burial beneath the sediment surface. Some coccoliths are preserved below the carbonate compensation depth. Many beds are bioturbated only in their upper parts. EXPERIMENTAL PROCEDURES 1. Visual observation of split cores and preparation of standard smear slides were employed. Detailed descriptions of typical cores are included in this paper. 2. X-radiography of 1to 6-mm-thick slabs of indurated sediment was utilized. Kodak Crystallex film was used; exposure times were from 1 to 7 minutes, at 3 ma and 25 to 40 kv. 3. Impregnations of wet samples were prepared by soaking them in a frequently replaced 50:50 mixture of acetone and methanol; then soaking in a 50:50 mixture of Araldite Ay 18 and HZ 18, diluted with an equal volume of the acetone-methanol mixture and a few drops of dibutyl phthalate. The acetone-methanol was allowed to evaporate slowly, and the sample then cured over a period of days gradually raising the temperature to 150°C. Some indurated samples were dried and impregnated with Canada balsam. Mr. M. Badcock prepared thin sections from the impregnations. 4. An E.E.L. photoextinction sedimentometer was used to determine the grain size of carefully selected small samples. Over 100 analyses were made. The theory of the method is described by Simmons (1959). The instrument used was similar in design to that of Jordan et al. (1971), but with a fall distance of 6.4 cm. and without rangechanging facilities on the galvonometer. Samples were treated with hydrogen peroxide (and where necessary in an ultrasonic bath) to disaggregate. They were wet sieved at 63 microns, and the sand fraction dried and weighed. The lutum was diluted sufficiently to by analysed. Calgon (0.1% solution) was used throughout as a dispersing agent. It was realised after most of the analyses had been run that with