Upper Jurassic and Cretaceous rocks of Taseko Lakes map-area and their bearings on the geological history of southwestern B C
Authors: Jeletzky J A , Tipper H W 
Published: February 2016

The tectonically complex, predominantly argillaceous Callovian to Barremian rocks of Tyaughton Lake map-area are 5, OOO to 9, OOO feet thick; they were named Relay Mountain Group. This group lacks lithological horizon markers and exhibits extremely strong and
irregular lateral facies changes. Its subdivision, correlation and structural interpretation are therefore only possible on a palaeontological basis. A number of palae ontological zones based on Buchia (=Aucella), ammonites and Inoceramus species was recognized in the predominantly marine, often
fossil-rich rocks of Relay Mountain Group. Most of these zones are recognizable all over Western Canadian Cordillera. The base of the group is invariably faulted. A regional unconformity appears to separate it from the overlying Jackass Mountain and Taylor Creek Groups. The Jackass Mountain Group
comprises as much as 15, OOO feet of greywacke, shale, and coarse boulder conglomerate of Aptian and (?)lower Albian age . The group is essentially non-marine and fossil flora occurs in places. The French Bar Formation of this group is a coarse boulder conglomerate apparently deposited at the base
of an essentially granitic mountain range; it grades rapidly westward into finer marine basal conglomerates of the Taylor Creek Group. The Taylor Creek Group is essentially marine Aptian and Albian black shale over 10, OOO feet thick. It is interbedded locally with thick chert pebble conglomerate
and with tuffs and volcanic breccias to the west. The group is believed to be the marine equivalent of much of the Jackass Mountain Group. The Kingsvale Group, over 15,000 feet thick, overlies the Taylor Creek Group conformably. This group is subdivided into four units: Division A of pebble and
cobble conglomerate, greywacke, shale and siltstone; Division B of andesitic and basaltic tuffs and breccias; Division C of volcanic conglomerate, greywacke and shale; and Division D also of andesitic and basaltic tuffs and breccias. Fossil flora in Division A indicates that the group is essentially
of Cenomanian and (?)later age. Late Middle Jurassic to Cretaceous rocks of Taseko Lakes map area were deposited in a long, narrow, northwest trending Tyaughton Trough (new name) limited by cordillera-like tectonic lands in southwest and northeast. These were intermittently uplifted and supplied
sediments to the Tyaughton Trough, which extended southeastward beyond international boundary. A sublatitudinal seaway connected Tyaughton Trough with the Vancouver IslandQueen Charlotte Islands Trough (new name) across northwestern Washington. Tyaughton Trough apparently was connected with the
Vancouver IslandQueen Charlotte Islands Trough northwest of Taseko Lakes map-area. Two more seaways must have been connecting the northwestern end of Tyaughton Trough respectively with the late Upper Jurassic and Lower Cretaceous seas of Northern Yukon and those of northeastern British Columbia.
These two seaways were permanently closed by the regional Aptian orogeny. The two above mentioned seaways across the present Coast Mountains were apparently closed permanently in the latest Albian time. Thereafter, only non-marine volcanic and sedimentary rocks were deposited in Tyaughton
Trough.