Abstract

Upper Triassic strata in the Trutch (94 G) NTS map area consist of the Charlie Lake, Baldonnel and Pardonet formations. These units comprise a mixed siliciclastic–carbonate succession, approximately 350 to 400 m thick. Sediments accumulated along a low gradient continental ramp deposited on the northwestern margin of Pangaea. Twenty-two lithofacies, deposited in distal offshore through supratidal settings are documented. Seven recurrent facies associations are identified in these strata; four reflecting deposition in continental to marginal marine settings, and three reflecting shallow to deep water marine deposition.

Facies association 1 (FA1) consists of interbedded dolomitic mudstone/siltstone and dolomitic sandstone that was deposited in an intertidal flat depositional setting. Facies association 2 (FA2) consists of normally graded, trough cross-bedded to current ripple-laminated lenticular beds of mudclast-rich fine- to medium-grained sandstone, and was deposited by a series of small tidal channels. Facies association 3 (FA3) consists of well-sorted fine-grained sandstone beds characterized by high-angle tabular to wedge-shaped cross-beds, low-angle planar cross-beds, low-relief, translatent ripples and inversely graded lamina sets. FA3 is interpreted to represent deposition by a series of elongate, shore-parallel aeolian dunes. Facies association 4 (FA4) consists of a heterogeneous succession of solution collapse breccias, rooted, pedogenically-altered mudstone and siltstone, and wave-rippled very fine- to fine-grained sandstone. It is interpreted as ephemeral lacustrine and/or supratidal sabkha.

Facies association 5 (FA5) consists of variably bioclastic quartzose sandstone, crinoidal packstone-grainstone and bivalve-dominated packstone beds. It is interpreted as a wave-dominated shoreface to foreshore on a shallow (inner) ramp. Facies association 6 (FA6) is dominated by very fine-grained calcareous sandstone and thin to thick-bedded bivalve-dominated wackestone, packstone and grainstone. It is interpreted as medial ramp (i.e., offshore transition to proximal offshore). Facies association 7 (FA7) consists primarily of organic-rich calcareous siltstone and mudstone (commonly with abundant carbonate concretions) and variably thick tightly packed bivalve packstone/grainstone beds. It is interpreted as outer ramp (proximal to distal offshore).

Siliciclastic sediment in the study area was derived from a variety of sources, including fluvial input from the east-northeast, longshore drift from the north, and aeolian input sourced from marginal marine dune fields.

Carbonate sediment in the study interval consists primarily of bioclastic detritus, although non-skeletal grains, particularly faecal pellets, and ooids are locally common. Skeletal carbonate material is dominated by disarticulated mollusc valves and crinoid skeletal debris but also includes ammonoids, articulate brachiopods, gastropods. Vertebrates (fish and reptiles) are also common.

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