Field geology under the sea with a remotely operated vehicle: Mona Rift, Puerto Rico

We implemented concepts of field geology at great ocean depths by constructing virtual outcrops from a string of overlapping video frames collected by remotely operated vehicles (ROV). This lower cost alternative to drilling boreholes allows stratigraphic extension into the offshore and for regional interpretation of marine seismic profiles. The imagery was collected along a dive transect on the west wall of Mona Rift, a deep and narrow rift NW of Puerto Rico between water depths of 1560 and 3927 m. The northern coast of Puerto Rico and its large offshore area are underlain by a mid-Eocene and younger forearc basin topped by a thick carbonate platform. There are no drillholes offshore and tying seismic lines across the shoreline there is problematic. We describe our virtual outcrop and constrain its age and stratigraphy using 7 ROV rock samples and compare them to deep boreholes and outcrops on land. Our formation descriptions and ages agree for the most part with those on land, but we identified a 100 m thick section, represented on land by an unconformity. Our stratigraphic interpretation indicates lateral variations in formation thicknesses and establishes a cross-section for additional sampling of the Eocene-Pliocene geology. It also suggests that Mona Rift has formed since mid-Pliocene. The presence or absence of Fe-Mn crust on rocks along the transect may be correlated with the smoothness of the rock surface.