42 



report and in greater detail in other sources (Fredette et al. 1990; Buller and 

 McManus 1979). 



There are a variety of grab type samplers of different sizes and design that 

 are used for collecting surface sediment samples. Most consist of a set of 

 opposing, articulated scoop-shaped jaws that are lowered to the bottom in an 

 open position and are then closed by various trip mechanisms to retrieve a 

 sample. Many grab samplers are small enough to be deployed and retrieved 

 by hand; others require some type of lifting gear. 



A simple and inexpensive dredge sampler can be made of a section of pipe 

 that is closed at one end. It is dragged a short distance across the bottom to 

 collect a sample. Unlike grab samples, the dredged samples are not represen- 

 tative of a single point and may have lost finer material during recovery. 

 However, dredge samplers are useful in areas where shells or gravel that 

 prevent complete closure of the jaws are present. 



Although obtaining surficial samples is helpful for assessing recent 

 processes, it is typically of limited value in stratigraphic study. Generally, the 

 expense of running track lines in coastal waters for the sole purpose of sam- 

 pling surficial sediments is not economically justified unless particularly 

 inexpensive boats can be used. Occasionally, grab and dredge samples can be 

 taken during geophysical surveys, but the sampling operations require the 

 vessel to stop at each station, thus losing survey time and creating interrupted 

 data coverage. Precise offshore positioning now allows grab samples to be 

 collected at specific locations along the boat's track after the survey has been 

 run and the data examined. 



Stratigraphic Sampling 



Sediments and sedimentary rock sequences are a record of the history of 

 the earth and its changing environments, including sea-level changes, 

 paleoclimates, ocean circulation, atmospheric and ocean geochemical changes, 

 and the history of the earth's magnetic field. On a global scale, the greatest 

 influences on the coastal zone are sea level fluctuations and plate tectonics. 

 By analyzing stratigraphic data, the age relations of the rock strata, rock form 

 and distribution, lithologies, fossil record, biopaleogeography, and episodes of 

 erosion and deposition at a coastal site can be determined. Erosion removes 

 part of the physical record, resulting in unconformities. Often, evidence of 

 erosion can be interpreted using physical evidence or dating techniques. 



Sediment deposits located across a zone that ranges from the maximum 

 water level elevation to the depth of the wave base are largely indicative of 

 recent processes. Within this zone in unconsolidated sediments, simple recon- 

 naissance field techniques are available for collecting data. The techniques 

 often use ordinary construction equipment or hand tools. Smaller efforts 



Chapter 3 Field Data Collection and Observation 



