AN EASTERN PROVENACE FOR THE NEW HAVEN ARKOSE AND THE PORTLAND FORMATION OF THE HARTFORD AND POMPERAUG BASINS: BROAD TERRANE REVISITED

Models of the provenance of Mesozoic basin sediments in New England call on eastward and westward transport of sediment forming the Late Triassic New Haven Arkose and the Early Jurassic Portland Formation in the Hartford and Pomperaug basins. The basins are half-grabens, bounded on the east by syn-depositional normal faults. Sediments most likely were shed from an escarpment to the east and from a peneplane to the west. Sediments derived from rocks to the east have Pennsylvanian (Alleghanian cooling) or younger white micas, whereas those from the west are all Mississippian (Acadian cooling) or older. Thus, the ages of detrital white micas can be powerful discriminators of provenance for sediments in the Hartford and Pomperaug basins. We tested this model using single-grain laser fusion 40Ar/39Ar and age spectrum dating of detrital white micas from eight Hartford basin and two Pomperaug basin outcrops. Age spectra from four outcrops of the New Haven Arkose show dominantly Alleghanian cooling ages. Single grain data from five samples gave exclusively Alleghanian cooling ages and two gave exclusively Acadian cooling ages. Age spectra from five samples from Portland Formation outcrops indicate ages ranging from 248 to 297 Ma and single grain data ranging from 250 to 310 Ma. These data show that Alleghanian white micas dominate the New Haven Arkose in the Hartford and Pomperaug basins, and overwhelm Acadian white micas in the Portland Formation. Therefore, provenance was dominantly from the east, and paleoslopes were dominantly to the west in the Late Triassic and Early Jurassic, even in western Connecticut. Thus, the eastern border faults of these two basins were indeed syn-depositional, but not in the extensional regime implied by the present eastern dip of these sediments. Rather, rocks under and west of the Hartford basin were more likely a foreland basin in a transpressional regime where rocks of the eastern New England hinterland (Bronson Hill terrane and east) had been continuously exhumed since late Pennsylvanian.