Petrology of Indurated Sandstones, Leg 18, Deep Sea Drilling Project

Abstract

Sandstones from Hole 177A (offshore north end of Vancouver Island) and Sites 178 and 181 (Gulf of Alaska) are compositionally immature feldspathic arenites containing abundant Plagioclase. Their compositions lie at the feldspar-rich end of the compositional spectrum for circum-Pacific sandstones. They probably were derived from andesitic-granodioritic-metamorphic arcs of typical arc-trench systems at times when plutons were greatly unroofed and volcanic activity was temporarily diminished.Sandstones from the Alaskan sites were cemented by calcite very early in diagenesis, which prevented further diagenetic alteration. They have very little silt and clay matrix, and the clays they do have are similar to those in associated shales. The calcite cement prevented grain compaction, so that the sand grains are openly packed in the cement.In contrast, sandstones from Hole 177A had a very complex diagenetic history. A possible 56-meter-thick sand body of probably Early Pliocene age may rest on acoustic basement about 520 meters below the sea floor in 2000 meters of water. The sand body is overlain by submarine-fan turbidites and hemipalagic muds, and may well be filling a submarine channel. Compaction of sand grains during basin subsidence and burial diagenesis reduced original porosity somewhat, but authigenic chlorite filling pores and replacing grains destroyed most of the porosity. After chlorite formation, calcite filled in the little remaining pore space and replaced some sand grains. Then the sediments at Hole 177A were uplifted, possibly because of the movements at the triple junction of the American, Pacific, and Juan de Fuca lithospheric plates where the site is located. The sand body and associated rocks were fractured in the process. Acid solutions of unknown origin permeated along hairline fractures and dissolved the chlorite and calcite cements, creating channels of intergranular porosity locally of 30 or 40 percent. Some of this solution porosity was filled later by precipitation of sparry anhydrite.