Abstract:

Deep-water turbiditic limestone of the Cambrian Hess River Formation in the Misty Creek Embayment (Mackenzie Mountains, Northwest Territories), a poorly known and economically underexplored part of the sedimentary exhalative/ clastic-dominant (SEDEX/CD)-hosting Selwyn Basin, contains a >20 m-thick black shale interval with elevated base metal and barium content and geochemical evidence of hydrothermal venting. Carbon isotope stratigraphy of the formation in its type area shows that its deposition in the Cambrian spanned from at latest early Stage 5 to mid-Paibian, and that its upper, conformable contact with the Rabbitkettle Formation, as originally defined lithostratigraphically, is coincident with the Sauk II-III boundary in the chemostratigraphic reference curve. The black shale addressed by this study was deposited in latest Stage 5, and a further ~50 m may exist under scree above the studied interval, probably extending into the early Drumian. This depositional age may be identical to that of host strata of mineralisation in the Anvil District, Yukon, but the depositional age of host rocks and mineralisation in the latter are not well constrained. The newly established depositional age of the Hess River Formation in the Misty Creek Embayment identifies significant dating and correlation problems in the broader Selwyn Basin. Biostratigraphic data from base of the overlying Rabbitkettle<br/>Formation in its type area indicate a Drumian depositional age, yet the top of the Hess River Formation in its type area is Paibian (i.e. 9 m.y. of overlap with depositional span of Hess River Formation in its type area). This conflict highlights the difficulty of mapping, picking contacts in, and correlating monotonous deepwater successions with little biostratigraphically useful content, subtle differences among formations, and diachronous changes in sedimentation regimes in a basin that was tectonically active. Although it is not field-friendly, carbon isotope stratigraphy may be the only viable tool that can be applied to deciphering the Selwyn Basin’s tectonic, stratigraphic, and metallogenic history.