Stratigraphy and tectonic significance of the Lower to Middle Devonian McAdams Lake Formation, Cape Breton Island, Nova Scotia

The McAdams Lake Formation consists of a lower member composed of green-gre y to black sandstone, siltstone, shale, and minor conglomerate, and an uppe r member composed predominantly of grey conglomerate and sandstone, with mi nor black shale. A lens of red shale, siltstone, and sandstone with minor c onglomerate is also present in the upper member. The total thickness of the formation is approximately 1 km. Fossil plant fragments and palynology ind icate an age of latest Emsian to Early Eifelian (latest Early to Middle Dev onian). The presence of a small, fine-grained quartz syenite intrusion prob ably led to the previously reported occurrence of volcanic rocks in the for mation. Characteristics of the lower member suggest deposition in a shallow lacustr ine shoreline setting with intermittent, rapid deposition of coarse sedimen ts in a fan delta near a faulted margin. The upper member is interpreted to represent an increase in topographic relief and a progradation of the fan delta to fill the basin, with braided channels present near the fan delta t oe. The presence of carbonaceaous shale in this member indicates intermitte nt swampy conditions or transgression of lacustrine conditions over the fan delta. The McAdams Lake Formation was deposited on the Mira terrane in a half-grab en formed by extension along the boundary between the Mim and Bras d'Or ter ranes, at the initial stage of development of the Sydney Basin. A hiatus of ca. 40 Ma occurred between deposition of the McAdams Lake Formation and th e unconformably overlying Grantmire Formation. During this time the McAdams Lake Formation underwent a period of compressive deformation that resulted in northeast-trending minor folds on an antiformal structure. The McAdams Lake Formation and equivalent units in Atlantic Canada may represent an ear lier Horton-like basin system with similar facies and economic possibilitie s that have been previously unexplored.