RIPOGENUS FORMATION, NORTHERN MAINE - AGE, SEQUENCE STRATIGRAPHY, AND SIGNIFICANCE OF SYN-DEPOSITIONAL TECTONISM

Detailed mapping reveals that a major sequence boundary (erosion surface) exists within the shallow marine strata of the Ripogenus Formation near the Ripogenus Dam in northern Maine. An orthoquartzite unit within the formation rapidly varies in thickness from 0-20 meters over distances of a few hundred meters. We infer that this rapid thickness change is due to the filling of paleotopography including significant paleorelief in the form of valleys. This erosion surface cuts into the distinctive "pitted" unit of the Ripogenus Formation, predominantly fine-grained calcareous quartz arenites, interbedded with lenticular limestone that is abundantly stromatoporoid-bearing. These lower strata of the Ripogenus Formation have yet to be paleontologically well-dated, at these exposures. A previously reported conodont-based age, of late Ludlow to early Lochovian, dates a bedded limestone member (a few meters thick) immediately overlying the orthoquartzite. An older regional study reported a possible mid-upper Llandovery age, from brachiopods, for the strata likely equivalent to the pitted arenite-limestone unit. Near the Ripogenus Dam the conodont-dated limestone is overlain by a thick (~50m) limestone breccia, which we interpret to be derived from mass wasting of post-orthoquartzite limestones, and initiated by syn-depositional normal faulting of the Ripogenus Formation. A section of ~100 meters of deeper-water mudrocks and interbedded pinstripe quartzite siltites (probable contourites), which overlies the limestone breccia, gives evidence of an event of coeval rapid subsidence. Mafic volcanics (West Branch) succeed the mudrocks and are also probably linked to the faulting and rapid subsidence we infer for the upper part of the Ripogenus Formation. This overall event sequence can be readily integrated with a model of flexural forebulge and outer foredeep normal faulting in connection with the start of the Acadian orogenic event in north-central Maine.