Sibley Group : a lithostratigraphic, geochemical, and paleomagnetic study

Abstract

The Sibley Group is an unmetamorphosed Mesoproterozoic red bed sequence, commonly flat lying, that formed in an intracratonic basin between 1450 Ma and 1500 Ma. It covers an area o f 15,000 km^, and reaches a total thickness o f 950 m. Recent drilling projects in the Nipigon Plate have provided a unique opportunity to study the basin using data obtained from a combination of 25 drill holes and surface outcrops. This has allowed a re-examination of the lithostratigraphy, as well as providing insight into the basin architecture. Previously, the Sibley Group consisted of only three formations: the Pass Lake Formation, the Rossport Formation, and the Kama HiU Formation (Cheadle, 1986a). Two additional formations have been introduced here; the Outan Island Formation and the Nipigon Bay Formation. The Pass Lake Formation, interpreted as a fluvial-lacustrine system, is divided into the Loon Lake Member and the Fork Bay Member (Cheadle, 1986a). The Rossport Formation is separated into the Channel Island Member, the Middlebrun Bay Member, and the Fire Hill Member (Cheadle, 1986a). These consist of cyclic dolomite-siltstone layers, stromatolites and red mudstone, which represent a playa lake, sabkha, and mudflat environments. The Kama Hill Formation is not subdivided, and is composed of purple shales and siltstones interpreted as subaerial mudflat deposits. The Outan Island Formation has been divided into the deltaic Lyon Member and the fluvial Hele Member. The Nipigon Bay Formation consists of cross-stratified sandstone beds, and is thought to denote an aeolian environment. Geochemistry has been used to examine formation-scale trends in weathering characteristics and provenance. Samples were collected from drill holes spanning the entire thickness of the Sibley Group, and the concentrations of various elements were plotted against depth to appraise variations between Formations. A paleomagnetic study was also conducted on the Sibley Group. The first part involved comparing paleopoles from samples of the Pass Lake, Kama Hill, and Nipigon Bay Formations. This revealed a probable depositional paleopole age between 1450 Ma and 1500 Ma, with remagnetization events at approximately 1350 Ma and 1100 Ma. The second part involved a study of paleo-secular variation in the Rossport Formation, which resulted in the documentation of one of the oldest known examples of paleo-secular variation yet discovered.