Trace fossils in diatomaceous strata of Miocene Monterey Formation: their character and implications
Younger parts of the Miocene Monterey Formation are commonly characterized by relatively unaltered diatomaceous strata. A common characteristic of these deposits is the preservation of varvelike lamination, indicative of deposition under anoxic or nearly anoxic conditions. Although laminated rock types are volumetrically dominant, bioturbated intervals are by no means rare, but little attention has been paid to the trace fossils themselves. Our study of trace fossils in the Monterey Formation demonstrated the significance of these biogenic structures in paleoenvironmental and paleoecologic analyses. In particular, trace fossils provide a means for detailed reconstruction of paleo-oxygenation conditions during Monterey deposition. Trace fossils in several Monterey sections exposed in central and southern California were examined in detail. At all localities, three major ichnofossil assemblages or ichnofacies were recognized: (1) Chondrites, (2) Planolites, and (3) Thalassinoides. The size and diversity of the three major ichnofacies and their lithologic associations suggest that the distribution of these facies is controlled primarily by the level of paleo-bottom water oxygenation. The Chondrites ichnofacies represents very low paleo-oxygen levels just above the anoxic threshold. The Planolites ichnofacies, with greater variety of larger burrow types, is indicative of slightly higher levels of oxygenation. Moderately to well-oxygenated conditions are suggested by the Thalassinoides ichnofacies. More detailed information on paleoenvironmental conditions can be gleaned by applying a refined trace-fossil tiering model. When used in detailed (centimeter-scale) vertical sequence analyses, this tiering model permits the translation of data on the composition, size parameters, and cross-cutting relationships of trace-fossil assemblages into relative paleo-oxygenation curves.
- Research Organization:
- Univ. of Southern California, Los Angeles
- OSTI ID:
- 6987379
- Report Number(s):
- CONF-8604187-
- Journal Information:
- Am. Assoc. Pet. Geol., Bull.; (United States), Journal Name: Am. Assoc. Pet. Geol., Bull.; (United States) Vol. 70:4; ISSN AAPGB
- Country of Publication:
- United States
- Language:
- English
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