Humble 

 Oil -A' 



Figure 6. Yearly mean sea-level series for four stations along the north- 

 central gulf coast. Data from tide gauges at Galveston, Eugene Island (at 

 the entrance to Atchafalaya Bay), Bayou Rigaud (Grand Isle), and Humble Oil 

 Platform "A" (13 km off Grand Isle). Data from Hicks and Crosby (1974) and 

 Baumann (1980). 



the area, although sea-level curves from as far away as Pensacola show a rapid increase 

 during the same period. In view of these rapid temporal changes, the predicted 

 subsidence rates in the following paragraph should be considered very tentative. 



From Humble Oil "A" and the Bayou Rigaud tide gauge records (Figure 6), one finds 

 a rate of local sea level rise of between 1.0 and I.I cm/yr for the period of duration of 

 the two records. By subtracting a rate of 1.2 mm/yr for eustatic rise, one arrives at a 

 subsidence rate of about 9 mm/yr for the south-central Louisiana coast. Farther west, at 

 Eugene Island, at the entrance to Atchafalaya Bay, one finds a subsidence rate of 7.3 

 mm/yr. A longer-term average subsidence rate can be derived from a C-based local 

 relative sea-level curve determined for the Caminada-Moreau beach ridge plain in 

 southern Lafourche Parish (Gerdes 1982). Gerdes' data suggest that local relative sea 

 level in that region has risen a total of 2.75 m during the last 1,000 years (Figure 7). This 

 corresponds to an average rate of 2.75 mm/yr. If one compensates for a eustatic rise of 

 1.2 mm/yr (assuming this rate to be valid for the last 1,000 years), then one finds a local 

 long-term subsidence rate of 1.55 mm/yr. This is a much lower rate than that derived 



171 



