ADIRONDACK GRABEN AND HALF-GRABEN: COMPLEX HISTORY OF FAULTING AND SEDIMENTATION

The Adirondack basement massif is transected by a network of NE (dominant) and NW trending lineaments that reflect deep differential erosion of fault zones. None are more obvious than the lineaments that trace through places like Long Lake, Indian Lake and Ohio gorge. Breccia, gouge and fracture zones are common features, and the structural blocks in between exhibit multiple sets of mineral-coated fractures. Detailed kinematic work revealed complex fracture abutting relationships, and very complex fault movement. Regardless the fault history, most exhibit normal displacement and the formation of graben or half-graben. In the central Adirondacks, there are several NE striking half-graben lined up E-W along the Prospect fault (PF), and they occur where the PF intersects major NE striking faults. The half-graben at Cedar River flow (CRF) and the Miami River line up with the Wells graben. Collectively, all five graben (and probably more) appear to form a regional en-echelon conjugate structure. They may be related to the rhombochasms south of the Adirondacks in the subsurface. The graben at Wells preserves Cambro-Ord. strata. Magnetic anomaly mapping shows the graben at Piseco Lake and half-graben at Miami River preserve Ordovician strata, while the half-graben at CRF is underlain by the local marble. The Miami River half-graben contains sediments derived from the regional gneisses and Paleozoic carbonates. Dolomite cemented conglomerate is comprised of fossil bearing limestone and granite gneiss pebbles, with garnet, pyrozene, quartz, feldspar sand. These rocks contain detrital zircons ranging in ages of 900-1900Ma (max population 1170) and 2540-2800 Ma. The graben received sediments derived from reworked Adirondack basement, and possibly the Superior Province. Earlier researchers showed that the faults were active during the development of the Cambro-Ord. Paleozoic self. Differential uplift of the Adirondacks in the Jurassic, most likely reactivated faults, producing graben and half-graben that preserve the Paleozoic sediments. At least one of the half-graben contain post-Paleozoic deposits, possibly Mesozoic or younger. But, it’s interesting to note that sediments that appear to post-date the Paleozoic rocks, in the Miami River graben, do not contain detrital zircons of Appalachian age or Mesozoic.