Origin of Lake Pontchartrain and the 1987 Irish Bayou Earthquake

Abstract

ABSTRACT The origin of the series of large estuarine lakes immediately north of New Orleans and the Mississippi river in southeast Louisiana are interpreted in light of a new interpretation of active faults which they are associated with. Within the largest of these lakes-Lake Pontchartrain-seismically identified, WNW-ESE striking, en echelon faults within the lake leads to the compelling conclusion that these faults are the continuation of the Baton Rouge/Denham Springs fault system. The BR/DS fault system has been previously mapped by surface fault traces extending from Baton Rouge to Slidell north of Lake Pontchartrain. Seismic data confirm this previous interpretation west of Lake Pontchartrain. However, east of the Manchac swamp seismic data demonstrate that the fault system is within the lake and not north of the lake-as previously mapped. There are no resolvable shallow seismic faults present along the previously mapped fault traces north of the lake. Further evidence that the faults within the lake are part of the active BR/DS fault system is the shallow seismic fault offsets within 800 feet of the lake bottom. Additionally subtle vertical offsets seen on the Hwy 11 bridge, the Southern RR trestle bridge, the Causeway (west span), and Interstate-10 twin span bridge correspond to the projected seismic fault traces. Other possible activity associated with these faults is the 1987 Irish Bayou earthquake. The fault segment which offsets the Hwy 11 bridge at North Shore dips southward toward the fishing village of Irish Bayou and probably intersects basement near the location of maximum observed intensity at Irish Bayou. The focal mechanism of the earthquake was probably generated by movement within the fault zone within basement. This suggests the possibility of a seismogenic fault zone which is 5 to 15 miles south and parallel to these faults and which extends across Lake Catherine, Lake Pontchartrain and Lake Maurepas and continuing toward Baton Rouge. The estimated magnitude of the Irish Bayou was 3.0 and would suggest that the seismic hazard along this trend is minimal. However, this is the initial identification of this apparently seismically active fault system. Until these faults are more closely monitored and their recent history more closely studied a more significant seismic hazard can not be entirely precluded. The location of an active fault system bounding Lake Pontchartrain, Lake Maurepas, and Lake Borgne is probably not a coincidence. Subsidence associated with this fault system enhanced if not triggered the present advancing transgression across the St. Bernard delta which is Lake Pontchartrain Lake Maurepas and Lake Borgne. This suggests the possibility that the St. Bernard delta may have completely filled the area occupied by these lakes. Fault movement may have created an initial open body of water which quickly is expanded by wave energy. This three stage model for the Lake Pontchartrain differs from previous hypothesis that Lake Pontchartrain is a passive feature being created by the southward closure of open sea by the modern Mississippi River and the older St. Bernard delta. The alternate model essentially requires the persistent and fortuitous bypass of the lake or ancestral bay by multiple deltaic cycles so that Lake Pontchartrain is eventually surrounded by deltaic deposition but is never significantly infilled by these deltas. The association of the lakes to the active fault system is unquestionable and suggests that the traditional passive model for the origin of the Lake Pontchartrain and its associated lakes do not fully account for the tectonic influence on the lakes. A second tectonic hinge is present in the Atchafalaya basin. This northwest-southeast zone along with the east-west tending Baton Rouge Denham Springs fault defines a zone of foundering crust in southeast Louisiana. This crustal instability started in the Early Miocene and has been triggered by the eastward encroachment along the Comanchean shelf edge of thick Tertiary sediments. Isostatic loading and flexural bending of the crust triggers or initiates faulting along the tectonic hinge of the Comanchean margin. These faults are the Baton Rouge/Denham Springs fault system. This zone of foundering crust has had two significant effects. One was to deflect the past Mississippi River deltaic courses southeastward, and the second is to trigger or enhance the transgressive lake system of Lake Pontchartrain, Lake Maurepas, and Lake Borgne. This zone of foundering crust has therefore affected two of most significant geomorphic elements in southeast Louisiana.