Sedimentology and depositional environment of the Middle Ordovician Black Cove and American Tickle Formations - western Newfoundland

Abstract

The Middle Ordovician (Darriwilian) Black Cove (Nicholsonograptus fasciculatus biozone) and overlying American Tickle Formations (Pterograptus elegans biozone) represent the lower portion of the Goose Tickle Group located in western Newfoundland. The succession consists of a total of seven lithofacies, four siliciclastic and three carbonate that are grouped into three distinct facies associations. Facies association 1 (FA1) contains intercalations of clay-rich mudstones (Facies A) with silt-bearing, clay-rich mudstones (Facies B) and in places, foresets of alternating siltstone and clay-rich laminae (Facies C). Facies association 2 (FA2) consists of rocks within FA1 and localized massive, silt-to-sandstones (Facies D). Facies association 3 (FA3) is characterized by carbonate mud-to-wackestones (Facies E), laminated and massive, peloidal, skeletal packstones (Facies F), and skeletal grainstones (Facies G). Each of the three facies associations is interpreted to represent a distinct position on a proximal to distal transect of a shelf that faced the proto-Atlantic. Bedload transport processes are present throughout the succession and are indicated by sedimentary structures such as ripples, planar laminations, mudstone rip-up clasts and lenticular siltstone laminae. These high-energy event deposits likely represent episodically occurring storms and are intercalated into fine-grained fair-weather sediments (Facies A, C, and E). The Black Cove and American Tickle Formations as a whole show an overall shallowing-upward trend that is subdivided into four coarsening-upward parasequences marked by carbonates (FA3) directly overlying fine-grained siliciclastic mudstones (FA1 and FA2). Each of these parasequences is interpreted to represent a lowstand unit attributed to a sea level fall. A comparison with time-equivalent lowstands worldwide suggests that at least two of these lowstands are most likely tectonically-induced. The presence of characteristic shelf sediments showing easily recognized sea level fluctuations, and the absence of turbidites within the Black Cove and American Tickle Formations suggests that these units reflect deposition in a distal shelf environment and not on a lower slope or within a basin as previously suggested. Phycosiphon incertum fecal strings and local Planolites isp. ichnofossils are abundant in the carbonate and fine-grained siliciclastic mudstone facies, providing evidence of dysoxic rather than anoxic conditions during deposition of the Black Cove and American Tickle Formations, allowing benthic burrowing organisms to flourish.