ORIGINAL ARTICLE Facies development of a Late Ordovician mixed carbonate- siliciclastic ramp proximal to the developing Taconic orogen: Lourdes Formation, Newfoundland, Canada Kelly L. Batten Hender Æ George R. Dix Received: 22 May 2007 / Accepted: 13 September 2007 / Published online: 19 October 2007 Ó Springer-Verlag 2007 Abstract The Upper Ordovician (late Whiterockian to Mohawkian) Lourdes Formation represents a narrow (tens of kilometers), short-lived [*5–7 million years (my)], open-ocean (high-energy) mixed siliciclastic-carbonate ramp that onlapped allochthonous strata along the orogen side of the local Taconic foreland basin. Platform devel- opment followed a 6–8 my hiatus during which weathering had concentrated chemically mature siliciclastics that were admixed with initial carbonate sediments. A cross-platform facies gradient contains paleokarst and peritidal carbonates and sandstones, shallow-ramp carbonate bioherms and skeletal shoals, and deeper ramp calcareous shales. Transgressive systems tracts are marked by ramp-wide sheets and shoals of skeletal grainstone and low accumu- lation rates, and highstand systems tracts are marked by significant admixture and interbedding of siliciclastics with cross-ramp carbonate facies. Platform demise coincides with increased siliciclastic input, which is likely tectoni- cally influenced. The Lourdes platform is equivalent to epicontinental foreland ramps along eastern Laurentia, but its narrower width precluded formation of oceanographi- cally restricted platform-interior facies. Keywords Mixed siliciclastic-carbonate facies Á Tectonics Á Lourdes Formation Á Upper Ordovician Á Newfoundland Introduction The Lourdes Formation (Long Point Group) of western Newfoundland, in eastern Canada, contains a detailed record of the birth, life, and demise of a Late Ordovician mixed siliciclastic-carbonate platform following westward overthrusting of basinal sedimentary and ophiolitic al- lochthons within a Taconic foreland basin. The platform represents a short-lived [5–7 million years (my)] re-estab- lishment of the carbonate factory, once a dominant sedimentary component of the older passive-margin and foreland successions in the region (James et al. 1989). Understanding facies development of the Lourdes For- mation is important because it lay on the opposite side of the foreland basin relative to the well-studied epicontinental platforms elsewhere in eastern North America (e.g., Holland and Patzkowsky 1998; Pope and Read 1997a); the platform developed in a still tectonically active setting; and envi- ronmental conditions may have created ecological pressure influencing emergence of the dominance of a reef con- structor guild more typical of post-Ordovician build-ups (Batten Hender and Dix 2006). Continuous (15 km) outcrop exposure along Long Point Peninsula (Fig. 1) provides the opportunity for assessment. This paper details the platform’s sedimentology, facies, and depositional history; presents a dynamic facies model; and underscores the depositional interplay of siliciclastic and carbonate sediments. Geological setting Regional geology The Humber Zone (Williams 1979) comprises the western Newfoundland portion of the Appalachian orogen. Its K. L. Batten Hender Á G. R. Dix Department of Earth Sciences and Ottawa-Carleton Geoscience Centre, Carleton University, Ottawa, ON K1S 5B6, Canada K. L. Batten Hender (&) PDI Production Inc, Suite 201, 10 Fort William Place, St. John’s NL A1C 1K4, Canada e-mail: kelly.battenhender@pdip.ca 123 Facies (2008) 54:121–149 DOI 10.1007/s10347-007-0126-0