--> Geobiologic Controls on Carbonate Sedimentation: Tanner and Cortes Banks, California Continental Borderland

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

Datapages, Inc.Print this page

Geobiologic Controls on Carbonate Sedimentation: Tanner and Cortes Banks, California Continental Borderland

Abstract

Tanner and Cortes Banks are shallow submarine banks in the outer California Continental Borderland, with highly calcareous, organic-rich surficial sediments. The fundamental controls on this - sedimentation pattern are: 1. Hydrodynamics (current and wave action) and, 2. Access to organic matter for benthic organisms. Tanner and Cortes Banks are shallow submarine banks in the outer California Continental Borderland, with highly calcareous surficial sediments, averaging 42% calcium carbonate on the bank tops, with fine organic material deposited on the proximal slopes and in an elongate saddle between the bank tops. The fundamental controls on this - sedimentation pattern are: 1. Hydrodynamics (current and wave action) and, 2. Access to organic matter for benthic organisms. These two fundamental controls are intimately interrelated and laterally variable. This bank top geobiologic niche is partly occupied by filter-feeding polychaete worms in the shallowest waters of the bank tops,. These polychaetes exhibit diverse feeding types and degrees of motility. Megaripples observed in bottom photographs and cross-bedding observed in shallow box cores both attest to rapid current and wave action on the bank tops. Rapid current and wave action are the oceanographic drivers for the filter-feeding polychaete food source-fine food particles kept continuously suspended. The off-bank (deeper water) geobiologic niche is characterized by finer sediments of the proximal slopes and bank top saddle. Surface deposit feeders occupy this niche, feeding on rich organic matter swept off the bank tops and deposited in fine-grained sediments. The spectrum of polychaete motility covers: 1. Sessile, 2. Discretely motile and 3. Motile. The spectrum is not itself related to any of the physical variables within the geobiologic niches, but the relative proportions of each category are related to the physical variables in these environments. Sessile polychaetes increase their proportions in deeper water as decreasing sediment motion reduces the chance of burial. Soft-bodied motile forms are ill-suited to the harsh bank top geobiological niche with its fast currents and sediment redeposition, but thrive in the relative tranquility of the proximal, fine-grained slope and bank top saddle settings. Discretely motile polychaetes are best adapted to the bank top environment, have protection in the form of mucous tubes and possess motility to avoid shifting sediments. The significance of this finding is twofold: 1. Previous researchers have only related deposit feeders to low energy, fine-grained sedimentary environments without differentiating between surface and subsurface deposit feeders and 2. Filter-feeding polychaetes robustly survive in areas of quite high current and wave reworking.