--> Biomarkers as Paleobathymetry Indicators: A Case Study from Miocene Marine Rocks, Central California, by P. G. Lillis; #90986 (1994).

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Abstract: Biomarkers as Paleobathymetry Indicators: A Case Study from Miocene Marine Rocks, Central California

Paul G. Lillis

Biomarkers are organic molecular fossils that are useful indicators of paleoecology, paleoenvironment, and paleogeography. Because of diagenetic and thermogenic alterations, it is often difficult to correlate a biomarker found in the geosphere with a specific source organism. Therefore, many biomarker indicators are interpreted by association with rocks of known paleoenvironments or paleo-ecosystems, as inferred by sedimentological and paleontological studies.

A comparison was made between some of the most frequently analyzed biomarkers (m/z 191, 217; terpanes and steranes, respectively) and the paleobathymetry and paleogeography of Miocene marine rocks from the Cuyama basin, California. The Miocene Monterey and Vaqueros Formations were deposited in a marginal marine basin at depths ranging from neritic to middle bathyal (0-1500 m). Multivariate techniques (principal components) were used to infer relationships between biomarkers and four bathymetry intervals. Results show that some biomarker parameters (hopane/sterane, pristane/phytane, diasterane/sterane, and Tm/Ts ratios) correlate to paleobathymetry because of their sensitivity to the geochemical conditions of the depositional environment. Other biomarker parameters (percent C27, C28, and 4-methyl

steranes) may reflect the relative input of algae from the overlying water column, which is related to bathymetry in some cases. Biomarkers that may be derived from benthic organisms thriving in specific bathymetric niches include certain tricyclic terpanes, extended hopanes, 28,30 bisnorhopane, and C24 tetracyclic terpane.

The use of biomarkers as bathymetry indicators is promising, but is significantly complicated by the dominance of allochthonous sources of organic matter from the overlying water column and from lateral transport. Future refinements may be made by using larger sample sets and additional biomarker compounds.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994