An offshore Mesozoic igneous sequence, Georges Bank basin, North Atlantic Available to Purchase

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.