Late Ordovician nearshore faunas and depositional environments, northwestern Maine

Abstract

The Little East Lake Formation represents a spectrum of Late Ordovician (Ashgill) nearshore environments. These physical environments are characterized by a variety of quartz- and feldspar-rich sandstone and slate. Depositional environments include neritic nearshore, beach, tidal flat, and alluvial(?). The beach and neritic nearshore environments contain a variety of fossil invertebrates. The majority of the brachiopod fauna is confined to two taxa:Eodinobolus rotundusHarper, 1984, andDalmanella testudinaria ripaeMitchell, 1978(inCocks, 1978). Some of the specimens have been broken and abraded suggesting transport within the beach swash zone. Gastropods includeLophospiracf.L. milleri(Hall),Lophospira(?),Trochonemellacf.T. notabilis(Ulrich and Scofield), andDaidia cerithioides(Salter). Tidal-flat environment contains the trace fossilsPalaeophycusandPlanolites.The Late Ordovician (Caradoc and Ashgill) sedimentary basins developed subsequent to the collisional Taconian orogeny, wherein an arc accreted to the eastern Laurentian margin. Prior paleomagnetic reconstructions place the southeastern continental margin of Laurentia at approximately 25° south latitude during the Late Ordovician. Using these reconstructions, the siliciclastic Ashgill rocks discussed here would have been deposited in an elongated, northeast-trending basin on the southeastern Laurentian margin. The fauna developed along this margin, but in contrast to possibly adjacent Irish and Scottish assemblages, was located in much shallower water.