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Late Middle Jurassic and Early Cretaceous mafic dikes, sills, flows, and local volcaniclastic sediments are intercalated within continental shelf sediments from the Baltimore Canyon Trough northward to the Grand Banks of Newfoundland. The igneous rocks on the eastern North American margin are mainly alkali basalts of intraplate affinity. The late Middle Jurassic igneous activity was of short duration, at about 140 Ma, and was restricted to Georges Bank where it led to construction of several volcanic cones. The main period of igneous activity was concentrated at about 120 Ma in the Aptian/Barremian. The activity consists of dike swarms in Baltimore Canyon, occasional dikes on the Scotian Shelf, and the growth of stratovolcanoes on the Scotian Shelf and Grand Banks. Younge dikes (~95 Ma) also are present on the Grand Banks.
The major phases of igneous activity are synchronous with major regional tectonic events, namely the initiation of rifting in the Labrador-Greenland region (140 Ma), separation of continental plates between Iberia and the Grand Banks (115 Ma), and separation of Labrador and Greenland continental plates (83-92 Ma). Most of the igneous activity occurred along old fracture zones that appear to have been reactivated and/or along fractures or fracture zones in the continental crust, resulting from changes in stress patterns caused by plate motion.
The co-occurrence of Middle Jurassic and Early Cretaceous volcanic activity on the continental slope off Georges Bank is evidence that a hot-spot process alone cannot explain the linear character and age trends within the New England Seamounts. Propagation of a fracture zone into oceanic crust more satisfactorily accounts for the age and distribution of the volcanism.
With regard to oil exploration on the continental margin, care must be taken to properly identify igneous and volcaniclastic rocks on mechanical logs, drill cuttings, and cores. Reflection seismic profiles can be used to map the areal extent of sills, flows, and low-angle dikes, which commonly show distinctive seismic responses. However, steeply dipping dikes generally produce little, if any, seismic response. Isotopic-age determinations of igneous rocks, combined with biostratigraphic-age determinations of adjacent strata, are invaluable for stratigraphic correlation, establishing chronology of seismic sequences, and analysis of basin sedimentation and tectonic history.
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