Paleoecology of unusually well-preserved west coast Upper Cretaceous fossil deposit: Point Loma Formation, Carlsbad, California
Macroinvertebrate fossils are best preserved in southern California Upper Cretaceous strata in the Point Loma Formation at Carlsbad, California. Large bulk-rock samples were removed from a construction site (now covered) that exposed the upper Point Loma at the Carlsbad Research Center. Collections made from a nearby roadcut of the Point Loma along El Camino Real were also studied. The dominant macroinvertebrate (31% of 978 individuals) is an undescribed aporrhaoid gastropod. Although aporrhaoids are present in most Upper Cretaceous deposits in North America, the abundance of these herbivorous mollusks within the Point Loma is unusual. Suspension feeders, including the bivalves Limopsis (28%), Indogrammatodon (5%), Corbula (5%), and Ostrea (4%), form the major trophic group present within this sample (total 57%). Deposit feeders and a diverse suite of predators form the remainder of the paleocommunity (total 4% and 8%, respectively). The two dominant taxa comprise the trophic nucleus of an aporrhaoid-Limopsis paleocommunity. Two taxonomically similar subpopulations can be distinguished within the studied Point Loma. At the El Camino Real site, sandy mudstones supported a subpopulation that attained full adult size. However, at the Carlsbad Research Center, homogeneous mudstones lacking sand yield diminutive fossils of the aporrhaoid-Limopsis paleocommunity. Ichnological evidence of a thixotropic mud bottom indicates that the size reduction was a result of sinking into the unstable substrate rather than other causes (e.g., low oxygenation or salinity). The thixotropic nature of the mudstone contributed to the exceptionally well-preserved fauna. Molluscan hard parts were rapidly buried. Bivalve ligaments, residual color banding, nacreous aragonite, and larval gastropods are preserved. After burial, the low-permeability mudstone insulated skeletons from carbonate dissolution.
- Research Organization:
- Univ. of Rochester, NY
- OSTI ID:
- 6987539
- Report Number(s):
- CONF-8604187-
- Journal Information:
- Am. Assoc. Pet. Geol., Bull.; (United States), Vol. 70:4; Conference: American Association of Petroleum Geologist Pacific Section convention, Bakersfield, CA, USA, 16 Apr 1986
- Country of Publication:
- United States
- Language:
- English
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