MOLLUSCAN FAUNA FROM THE PERMIAN KAIBAB FORMATION, WALNUT CANYON, ARIZONA

During the summer of 1946, field investigations of the Alpha member of the Permian Kaibab formation were undertaken in the region of Walnut Canyon, south of Flagstaff, Arizona. Excellence of exposures greatly facilitated the measuring of sections and tracing of rock units along the canyon. Specimens were collected for lithologic analyses, and representative fossils secured from several zonules.

More extensive fossil collections were made at three exposures several miles north of Walnut Canyon, and at a fourth some 35 miles north. The fauna is analyzed and described, and an attempt is made to interpret the environment in which it developed and the conditions under which it was preserved. Environmental factors such as depth, salinity, temperature, and turbidity of the water, presence or absence of currents and other agitation, intensity of light, and the nature of the bottom are discussed. Conclusions reached indicate that the fauna, composed largely of mollusks, developed in a warm, shallow, epeiric sea not very far from land. A gradual retreat of the sea probably did not modify the depth of the sea very greatly. From lowland to the northeast, fine sediments were brought into the sea, but it is believed that the water was not very brackish, except perhaps for short periods of time. The sea bottom was composed of a silty, clayey, calcareous mud containing abundant shells of dead organisms, and most of the smaller mollusks may well have lived attached to seaweeds and other marine growth, while larger and heavier forms crawled over or burrowed into the mud bottom. Periodic changes of conditions, probably due to oscillations in depth and a corresponding change in the nearness of the shore, brought about local extinction of the fauna and changes in the nature of the sediments.