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Description
The Middle Eocene Ardath Shale is part of a transgressive-regressive sequence which makes up the Middle Eocene sedimentary deposits in the San Diego area. The formation is regarded as the marine continental shelf facies. Molluscan fossil assemblages and sedimentary structures are abundant in parts of the formation and it is feasible to divide the formation into a lower, middle, and upper part on the basis of these characteristics. The lower part is characterized by the scarcity of molluscan fossils and beds which are laminated to thin bedded. The middle part contains abundant nonbiogenic sedimentary structures such as slumps, channel structures, and a complete or partial sequence of graded beds, parallel laminae, wavy and cross-laminae and parallel laminae confined to the sand-stone beds. The upper part is marked by abundant molluscan fossils, and local bioturbation of the sediments. Nonbiogenic sedimentary structures are predominantly laminae. This division is only tentative as the monotonous lithology and absence of marker beds in the formation makes correlation over extended distances very difficult, if not impossible. Based on the examination of sedimentary structures and lithologies, it appears that the sediments of the Ardath Shale were deposited in environments similar to those which exist offshore of San Diego at present. The sediments were accumulated first on the continental shelf. Some were partially remobilized and funneled down a canyon system, then redeposited and reworked on a submarine fan and trough at the end of the canyon. The sedimentary structures found in the Ardath Shale are the result of a slumping and periodic reworking by low energy bottom currents. A depositional mechanism, such as turbidity currents, is ruled out in favor of one which involves intermittent deposition and reworking by bottom currents. Paleoecological investigations of the molluscs of the Ardath Shale indicate that depths in which the assemblages were deposited were varied, from as shallow as 30 feet to as deep as 200 feet. A warm climate (with surface temperatures of greater than 20° C) is suggested as most of the molluscan genera have tropical affinities. The molluscan assemblages represent fossil communities which have undergone little or no preburial alteration by currents. Biogenic sedimentary structures also indicate that the fossil-bearing sediments were deposited in a shallow neritic environment.
