Depositional history and stratigraphy of the Aphibian Tamarack River Formation and the Paleohelikian Sims Formation, western Labrador

Abstract

Alphebian sedimentary rocks of the Tamarack River Formation rest unconformably on the Knob Lake Group in the south-central portion of the Labrador Trough. The formation appears to consist of two transgressive-regressive cycles. The first cycle, present in Member A, consists of subtidal siltstone, which grades upward into a mixed siliciclastic-carbonate unit of shallow subtidal and intertidal origin. The second cycle, recorded by members B and C, is composed of subtidal siltstone which passes upward into the intertidal and fluvial sandstone. Detritus of member A was derived from the north, whereas the provenance of member C was the Superior Province to the west. The formation may be part of the third Aphebian miogeoclinal sequence which developed in the Labrador Trough. The first and second sequences are recorded by the Knob Lake Group. The northern Labrador Trough may contain third sequence strata correlative to the Tamarack River Formation. -- The Paleohelikian Sims Formation rests unconformably on deformed Aphebian strata and consists of a lower unit of terrestrial redbeds and an upper unit of fluvial to shallow-marine quartzarenite. The relatively thin terrestrial redbed unit is composed of three facies, a talus-slope breccia, a pebbly braided-river conglomerate and a sandy braided-river deposit. These facies grade upward into a thick, tabular quartzarenite unit composed of fluvial sandstone, beach conglomerate, and nearshore to shallow-marine sandstone. Throughout its deposition the formation progressively onlapped from west to east and derived its detritus from an uplifted area to the east. The formation may represent post-Hudsonian molasse, and is possibly correlative with the lithologically similar but undated Sakami Formation of northern Quebec. -- Both the Tamarack River and Sims Formations were intruded by tholeiitic dikes and sills of the Elsonian (1.4Ga) Shabogamo Intrusive Suite. Paleomagnetic samples from the suite yield a virtual paleopole position at 3.97°E, 3.4°S with dp = 5.8° and dm = 11.0°, which is similar to the antipole of the nearby Elsonian Michikamau Intrusion.