Lateral Continuity of Facies and Parasequences in Middle Cambrian Platform Carbonates, Carrara Formation, Southeastern California, U.S.A.

Abstract

ABSTRACT Lateral and vertical changes within a 40-m-thick interval of peritidal carbonates (Middle Cambrian Jangle Limestone Member, Carrara Formation, SE California) produce a mosaic pattern of laterally continuous and discontinuous facies, shallowing-upward cycles 2-20 m thick (parasequences), and surfaces bounding parasequences. This mosaic pattern was traced, bed by bed, across 1.8 km of continuously exposed outcrop, and then correlated to sections in nearby ranges. Within parasequences, contacts between successive subtidal facies tend to be sharp but may pass laterally into gradational contacts. Most subtidal facies extend laterally for less than 300 m, passing gradationally (over tens of centimeters to meters) into other, laterally adjacent subtidal facies. Lateral facies contacts between subtidal nodular ribbon rock and intertidal fenestral limestone are gradual over a few meters, whereas contacts between vertically juxtaposed facies generally are sharp. Parasequence boundaries generally are sharp and locally show several tens of centimeters of erosional relief over lateral distances of several meters. Most parasequences and their bounding surfaces extend for the 1.8 km of outcrop studied and correlate to nearby ranges with varying degrees of confidence. Several parasequences, however, do not extend laterally for more than a few hundred meters and are not distinguishable in a vertical profile from those that extend laterally for tens of kilometers. The stratigraphic relationships at Eagle Mountain (southeastern California) support a model in which the lateral continuity of parasequence surfaces is principally determined by accommodation variations, and the lateral continuity of facies is most strongly influenced by the intrinsic dynamics of spatially and temporally heterogeneous sediment production. The presence of parasequence boundaries of highly variable lateral extent suggests that the magnitude of accommodation variations was comparatively small, consistent with the independent evidence for passive-margin subsidence and transitional greenhouse conditions for the Middle Cambrian.