NEW CORES RESOLVE AN OLD GEOCHONOLOGIOCAL CONUNDRUM FOR THE CENTRAL ATLANTIC MAGMATIC PROVINCE (CAMP) EXTRUSIVE ZONE IN THE NEWARK, HARTFORD, AND DEERFIELD BASINS

The new Metropolitan District Commission (MDC) geotechnical cores (Hartford, CT) solve a 35 year geochronological conundrum in the astrochronology of the syn-CAMP lacustrine sequences of the Newark, Hartford, and Deerfield basins. The remarkable similarity of the details of the lacustrine cyclostratigraphies in the extrusive zone of the three basins has been known in basic outline since the early 1980s. In 2007 we used a age model of cyclicity based on the underlying Triassic age sequences to obtain a total duration of about 610 ky which was later tested and corroborated by zircon U-Pb dates from the basalts and associated intrusions.

A consequence of this age model was that, while the oldest and youngest flows in the Newark and Hartford basins appeared to occupy gaps of no more than 30 ky total, the middle flows appeared to represent a gap of nearly 60 ky. But, while the Newark Basin extrusive zone was known from nearly continuous core, that of the Hartford Basin was not. The limited outcrops of the lower-third of the East Berlin Fm (Hartford Basin) could not be unambiguously connected to the upper 2/3 of continuous exposures, but barring contrary data, we assumed that astrochronology of the lower East Berlin Fm. plus Holyoke Basalt was the same as seen in the Newark Basin. Two anomalies were evident: first, the lacustrine cycles from the base of the East Berlin seemed unusually well developed; and second, the cyclostratigraphy of the lower Turners Falls Ss at Turners Falls in the Deerfield Basin seemed to project below that of the Newark and Hartford basins with the consequence that the Deerfield Basalt was equivalent to a gap of less than 20 ky.

The MDC cores span the entire East Berlin Fm and resolve this problem in a remarkable way. The cyclostratigraphy of the upper 3/4 of the formation matches the Newark Basin nearly perfectly, but a well-developed series of cycles extend downward from the correlative sections compensating for the upper two flows of the Preakness Basalt. This new astrochronology fills the temporal gap represented by the middle CAMP flows and now the underlying celestial mechanical information can be derived empirically from the section. How exactly sedimentation in the Hartford and Deerfield basins “knows” how to compensate for the gap represented by the middle flows is now the residual mystery.