Abstract

In 1981, a Georges Bank outer continental shelf oil and gas exploration well (Exxon Block 133-1), between the COST G-1 and G-2 wells, penetrated a 341-m (1,120-ft) sequence of igneous rocks interbedded with Middle Jurassic limestones. Interpretation of geophysical well logs combined with core and cutting samples resulted in recognition of two distinct units: (1) an upper 225-m- (740-ft-) thick interval of volcaniclastic rock inter-bedded with limestones, which we interpret as a remnant Middle Jurassic volcanic cone, and (2) a lower 76-m- (250-ft-) thick basalt that has yielded radiometric age dates of ∼136 Ma, which we interpret as an Early Cretaceous intrusion.

Chemical data that were obtained from core samples of the lower basalt unit indicate that it is SiO2 undersaturated (avg. 44.24 wt.%), high in K2O (avg. 1.66 wt.%) and Na2O (avg. 3.44 wt.%), and particularly high in TiO2 (avg. 3.99 wt.%), P2O5 (avg. 1.24 wt.%), and Zr (584 ppm). The basalt is a nepheline-normative alkalic type.

Comparison of our offshore chemical data to onshore petrologic groups shows that the lower 76-m- (249-ft-) thick Georges Bank basalt (GBB) most strongly corresponds to the New England–Quebec (NEQ) alkalic province, both chemically and temporally. Although we are not suggesting a genetic relationship between the GBB and the NEQ province, we do recognize the GBB as a coeval offshore episode of alkalic activity that is petrologically distinct from the Early Jurassic eastern North American province. The GBB also appears to be part of a widespread eastern North American Late Jurassic–Early Cretaceous alkalic episode.

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