Abstract

In any body of standing water subject to wave action, three depositional environments can be distinguished. They are here named the unda, the clino, and the fondo environments. In each, conditions are distinctive, but within each relatively uniform. The physical agencies operative in each of them, together with the kind and rate of sedimentation and rate of subsidence, control sedimentary facies and produce distinctive features of composition and bedding which are criteria for determining the depositional environments.

Wave base, the greatest depth to which the bottom is stirred by waves during storms, is a critical factor. Above it, on the undaform, the sediments are relatively coarse, generally fossiliferous, rippled and cross-bedded, and show a peculiar “wavy” type of relatively thin bedding. The clinoform is the sloping surface extending from wave base down to the generally flat floor of the water body, here called the fondoform. In the clino environment—i.e., that on the clinoform—mud is dominant. Fossils, except for microforms in the later sediments, tend to be scarce. The bedding shows a distinctive alternation of thin and remarkably even layers of silt- and clay-sized material. Flute markings, probably produced during storms by density currents, and various indications of slumping and flowage of and in the sediments are common. The fondo environment is characterized by fine material with even, uniform, and generally rather massive bedding, but uniform, thin-bedded alternations of silt and shale without flute markings may develop close to the base of the adjoining clinoform.

Changes in sea level shift these environments and produce characteristic sequences of strata. One inferred effect of a drop in see, level is the deposition, all along the outer edge of the undaform, of a belt of sands—here called undaform-edge sands—capable of becoming reservoirs for oil and gas. Such sand zones are likely to be misinterpreted as “shoreline trends.”

Rocks deposited in the unda environment are relatively coarse, being most commonly sandstone, coarse siltstone, fragmental limestone, or oölite. The bedding is distinct and commonly has a characteristic waviness. Ripple marks, cross-bedding, flow-and-plunge structure, and lenticularity are characteristic.

Criteria for clino rocks are: texture fine enough for the material to have been carried in suspension, bedding relatively thin and remarkably even, and the presence of flow markings and flutings and of evidences of gravity sliding.

Criteria for fondo rocks are: fine grain, lack of secondary sorting, generally massive bedding, and a composition generally derived from clays and oozes.

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