Trace fossils record the physical record of the activities (burrowing, crawling, feeding) of sea-floor organisms, as opposed to body fossils, which represent shelly remains. Unlike body fossils, trace fossils have limited geologic age information but are valuable as indicators of certain depositional environments/sequences. It is pertinent to mention that the scientific names of trace fossils follow the same rules of the international "Code of Zoological Nomenclature" that is used for body fossils.
This post highlights several of the genera of trace fossils that occur in a well exposed section of submarine-canyon-fill deposits in the Paleocene Carmelo Formation, coastal central California, Monterey County, northwest of the town of Carmel. The submarine canyon was carved into a granodiorite basement rock of Cretaceous age and eventually filled with deposits of relatively deep-marine origin.
The trace fossils are prolific and are confined to slope-channel overbank deposits consisting of finer sandstone and mudstone interbeds that occur within a conglomeratic stratigraphic vertical succession.
Several of the more common trace fossils found in the Carmelo Formation are illustrated below.
Ophiomorpha burrows are common, range mainly from Jurassic to Recent, and commonly have pellet-lined tubes. These pellets are fecal material secreted by the burrowing-shrimp animal that digs the burrow and thereafter packs this fecal material into the walls of the surrounding sediment in order to strengthen the walls. These kinds of burrows can be horizontal or vertical, depending on the rate of sedimentation. Horizontal Ophiomorpha burrows are commonly found in areas of relatively slow sedimentation, whereas vertical burrows are commonly found in areas of relatively rapid sedimentation, where the shrimp had dig rapidly and quickly move upward in order to prevent from being buried alive by a sand flow.
Scolicia burrows are common, non-marine to marine, and range in age from Cambrian to Recent. They are meandering horizontal trails, bilaterally symmetrical (ribbon-like), and about 1 to 5 cm wide. They have numerous very thin (meniscate) partitions that can be variably spaced. These partitions which probably represent resting stages of the burrower.
Hillichnus loboensis burrows are rare and known only from Paleocene to middle Eocene marine rocks in California. This trace fossil is large, with a complex feathery form. They were first found in the Carmel Formation, but this trace fossil (interpreted to be made by a burrowing bivalve) has been reported also from middle Eocene shallow-marine deposits in the San Diego area. A camera-lens cap (about 2.5 inches in diameter) is used for scale.
Chondrites burrows are very common, widespread, and known only in marine deposits (usually deep-marine turbidites) of Cambrian to Recent age. They are small and branching, with a plant-like expression on bedding planes. In the Carmelo Formation, they occur as white, tiny branching structures found in muddy beds probably deposited in low-oxygen environments. In this slide, the small whitish Chondrites burrows occur in the brown layer directly above the point of the ball-point pen.
Unidentified A burrow: large, three-dimensionally preserved, and foliate (possibly a new genus). A ball-point pen is used for scale.
Unidentified B burrow: wispy and meniscate (possibly a Scolicia?).
A ball-point pen is used for scale.
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