Abstract

Two intervals of Upper Ordovician organic-rich (gas-prone) source rocks are preserved in a regional fault block of approximately 1000 km² within the Ottawa Embayment (eastern Ontario). They bracket an apparent conformable mid-Katian (late Edenian to early Maysvillian) transition from a relatively shallow-water carbonate platform setting to a deep-water shale basin within the local Taconic foreland basin. The platform succession is represented by the Lindsay Formation, and records deepening and replacement of a once skeletal-rich, productive platform environment by fossiliferous shale and limestone facies association (10 to 18 m thick) comprising the Eastview Member. Nodular and bedded crinoidal lime mudstone is intercalated with fossiliferous (trilobite, brachiopod) organic-rich (2–6% TOC) shale. The upper 2 m of the unit contain shale with dispersed to compacted accumulations of trilobites and brachiopods. The Eastview Member thickens to the southeast, and is overlain with apparent conformity by organic-rich (2–4+% TOC) shale of the Billings Formation. Regionally, this latter unit is approximately 20 m thick, and accumulated in an anoxic to dysoxic deep-water basin. Thin laminae of siltstone to fine-sandstone herald the later, principal, phase of basin fill by sandstone and shale that characterize the overlying Carlsbad Formation. Geophysical (gamma-ray) transects restricted to the western embayment reveal km-scale spacing of high-angle faults and small-scale synsedimentary horsts and grabens that formed after shallow (3–5 m) burial of the Eastview Member. One small-scale graben illustrates a near doubling (<40 m) of the Billings Formation. A tectonostratigraphic framework for source rock accumulation includes a first-order geodynamic control driving regional platform foundering, then a higher-order structural response that enhanced local thickness of organic shale and created faults subject to reactivation in post-Ordovician time.