The presence of gray layers, representing flood deposits, has previously been well documented in sediments obtained from Santa Barbara Basin, California. We studied geochemical aspects of these layers, in particular the carbon and sulfur contents and isotopic compositions, and found evidence of rapid pyrite formation. The data are consistent with complete (or nearly complete) sulfate consumption within the pore waters of a gray layer. This requires sulfate reduction rates (SRRs) that exceed the rate of resupply by diffusion, thus on the order of milimolar per day. The apparent rapidity of this diagenetic reaction is addressed. The net result is the precipitation of authigenic pyrite crystals enriched in ³⁴S that become part of the sediment record. Thus, any interpretation of S isotope fluctuations in the marine sedimentary rock record obtained from pyrite must take into account the phenomena we describe and document here.