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Relationship Between Relative Sea Level Fluctuations and Petrologic Variation in Forearc Basin Sedimentary Rocks of Upper Cretaceous Rosario Formation at San Carlos, Baja California del Norte, Mexico

William R. Morris, Cathy Busby-Spera

Petrographic study of a well-exposed submarine canyon fill sequence indicates that petrologic variation corresponds to changes in relative sea level. These changes are recorded as textural variation in the canyon fill: a basal coarse-grained unit capped by a middle fine-grained unit records a rise in sea level, and an upper coarse-grained unit records a temporary fall in sea level. These sea level fluctuations do not correlate with published eustatic sea level curves, although the temporary fall may be recorded on a regional (San Diego to San Carlos, 400 km) scale. Sandstone samples from the lower and upper coarse-grained units contain higher concentrations of volcanic rock fragments and plagioclase than the middle fine-grained unit, whereas the middle fine-grained unit c ntains higher concentrations of quartz and K-feldspar.

Fluctuations in relative sea level may have been controlled by tectonic or magmatic events: (1) a tectonically controlled rise in relative sea level could have resulted in longer residence time of the sediment in a nearshore environment, causing increased mechanical and chemical weathering of the least-resistant grains (i.e., plagioclase and volcanic fragments) relative to quartz and K-feldspar; or (2) a temporary lull in volcanism could have resulted in increased dissection of the arc and decreased sediment input due to lowered base level within the arc, resulting in a marine transgression and an increase in the concentration of quartz and K-feldspar in the sediment. The lack of significant alteration of the feldspar grains in the fine-grained unit relative to the coarse-grained unit may favor the second hypothesis.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.