Oceanography and Geology of the Gulf of the Farallones 



Sediment of the Sea Floor 



Herman A. Karl 



Many people perceive the sea floor to be a 

 smooth blanket of sand similar to a sandy 

 beach. For some areas of the sea floor 

 this is true, but just as the sandy beach 

 is flanked by rocky headland and muddy 

 wetland, so are the smooth sandy plains of 

 the sea floor flanked by various different 

 substrates. This is the case for much of the 

 sea floor in the Gulf of the Farallones. 



The earliest general model of sedi- 

 ment distribution across the sea floor 

 was that the size of sediment particles 

 gradually decreased with increasing 

 water depth. According to this model, 

 the nearshore consisted of a blanket of 

 sand an extension of the sandy shore 

 that was gradually replaced by silt and 

 then by clay in deeper water. This model 

 was based on the concept that the energy 

 of ocean currents and waves decreased 

 from shallow water to deeper water and 

 that, therefore, smaller particles could 

 only settle to the bottom in less ener- 

 getic, deeper water. 



This early model was based on very 

 limited data. These limited data led sci- 

 entists to believe that the deep ocean 

 floor, from which very few samples had 

 been collected and of which no direct 

 observations were possible, consisted of 

 a uniform and monotonous layer of fine- 



grained sediment. As sampling tech- 

 niques improved and more information 

 was collected, scientists learned that the 

 ocean bottom is as texturally varied and 

 morphologically complex as the land 

 surface. The physical characteristics (for 

 example, size, shape, composition) and 

 the distribution of sediment are the 

 result of a complex interaction among 

 geologic, oceanographic, and biologic 

 processes. Moreover, scientists know 

 that the distribution of sediment on 

 the Continental Shelf is made more 

 complex by deposits of relict sediment. 

 Relict sediment is that material, gener- 

 ally coarse sand and gravel, left on the 

 shelf when it was exposed during times 

 of lower sea level. 



Evidently, then, the distribution of 

 different types and grain sizes of sed- 

 iment and rocks provides clues about 

 the geologic history of an area and the 

 types of ocean currents that deposited 

 or reworked the sediment. Moreover, the 

 different substrates provide habitats for 

 the various organisms that live on, in, 

 and near the sea floor. Information about 

 substrates in Gulf of the Farallones 

 National Marine Sanctuary is used by 

 scientists investigating biodiversity and 

 ecologic systems to help understand and 



manage the variety of animals and plants 

 that live in the sanctuary. Such informa- 

 tion is also used by commercial fisher- 

 men as an aid for locating fishes and 

 crabs that prefer a particular substrate. 



The sea floor in the Gulf of the 

 Farallones consists of many different 

 types of substrate, including rock out- 

 crops, gravel, sand, clay, and deposits of 

 broken shells. Some of these different 

 types of substrate are found very close 

 to each other and have abrupt boundar- 

 ies between them. On the Continental 

 Shelf a wide corridor of sand extends 

 westerly from the Golden Gate to the 

 Farallon Islands. Silty sand and sandy 

 silt bound the corridor to the northwest 

 and southeast, and a band of silt extends 

 around Point Reyes. 



In general, the sediment on the sur- 

 face of the Continental Slope is finer 

 and more uniform than that on the Con- 

 tinental Shelf in the Gulf of the Faral- 

 lones. Nonetheless, most of the surficial 

 sediment on the Continental Slope in the 

 area of the Gulf of the Farallones stud- 

 ied is very sandy, a condition that is 

 unusual (continental slopes are gener- 

 ally characterized by silt and clay). The 

 reason for this abundance of sand is 

 not fully understood. In general, excep- 



tionally strong currents are required to 

 transport large amounts of sand from the 

 shelf to the slope, but such situations are 

 extremely uncommon. 



It is not yet known whether the 

 textural patterns and bedforms on the 

 Continental Shelf in the Gulf of the 

 Farallones reflect entirely present-day 

 processes or whether some of these fea- 

 tures are remnants of processes that 

 operated during lower stands of sea level 

 in the past. During the glacial cycles of 

 the past several million years, sea levels 

 were lowered to as much as 445 feet 

 below present-day sea level. Therefore, 

 ancient relict features created during 

 lower stands of sea level are common 

 on Continental Shelves worldwide. Such 

 lowstands of sea level would have 

 exposed virtually all of the shelf in the 

 Gulf of the Farallones. 



24 Oceanography and Geology of the Gulf of the Farallones 



