The Upper Triassic Sinwa Formation provides one of the most useful horizon markers in northwestern British Columbia. It consists almost entirely of grey, usually petroliferous, white-weathering lime- stone and varies in thickness from a couple of metres to more than 600 metres. It is a wide spread occurrence and forms a continuous band along the north side of the King Salmon thrust fault, extending from Sinwa Mountain southeast to Kowatua Creek, subparallel to the Inklin River.

Well-preserved Upper Triassic fauna was found and identified within the Sinwa Formation. Most of these collections were examined by the Geological Survey of Canada and were reported as Norian (Late Upper Triassic) Age. Most of the limestone contains schleractinian corals which indicates they are Triassic or younger.

In the Tulsequah area, the Sinwa limestone has served as a plane, along which extensive thrust faulting, accompanied by intense local folding has occurred. The principal fault, the King Salmon thrust fault, corresponds closely with this main belt of limestone. The direction of movement has been from northeast to southwest, and a small klippe of Sinwa Formation resting on Lower Jurassic Laberge Group strata east of Trapper Lake (Kowatua Creek 104K 070) indicates a minimum displacement of about 16 kilometres.

The massive limestone is reported to carry carbon and some bitumen. Near the base of the formation 0.6 centimetre thick inter- beds of coal and chert are present on Sinwa Mountain.

Near Kowatua Creek the Sinwa limestone lies immediately above the King Salmon thrust fault and is variably silicified, brecciated, and contain veinlets of fluorite (Tardis 104K 112).