Abstract

Sediments of limestone depositional environments are uniquely rich in the secreted skeletal carbonates of marine organisms, clearly attesting to the great importance of animal and plant life in the formation of most carbonate rocks.

By far the outstanding example of organic activity in any sedimentary environment is the construction of reefs or mounds that are built, distinctively and wholly, in response to a localized intense concentration of organisms. The structures thus built are irregular in shape and size, and are typically massive in gross appearance, lacking the stratification or bedding of the surrounding sediments. The specialized environment at the reef site is above-all characterized by a distinctive abundance of organisms, general absence of bedding, and an implied but invariable concept that the upper growth surface of the reef lies above the surrounding sediment floor. Differences in the magnitude of growth relief distinguish between reefs on the one hand and mounds or bioherms on the other. Both types are valuable in providing reservoir rocks for petroleum, but in entirely different ways.

Stromatolites likewise are manifestations of intense organic activity. They are bedded layers that consist generally of laminated carbonate sediment entrapped by blue-green algae. These layers, normally not exceeding 5 or 10 feet in thickness, are interbedded with other kinds of limestones and are a common element of many stratigraphic sequences. They are valuable as indicators of the intratidal or extremely shallow-water-marine environment, but in spite of their complete dependence upon organisms, they play no role as reservoir rocks in the entrapment of petroleum.