Abstract
An unusual Upper Triassic reef complex that is thought to have formed in a forearc basin is exceptionally well-exposed at Lime Peak in the southern Yukon. This exposure offers a special opportunity to study both Triassic reef-building in North America and reef development in a convergent margin.The reef complex at Lime Peak consists of a variety of superimposed reef mounds and bedded inter-mound limestones and is surrounded by volcaniclastics. Unlike previously described Triassic buildups in North America, which are generally thin biostromal deposits dominated by corals and spongiomorphs, the reef mounds at Lime Peak are up to 150 m thick and the principal framebuilding fossils are small calcareous sponges. In addition to framework, significant amounts of lime sand and mud are also present in the mounds at Lime Peak. These sediments are interpreted to be locally produced accumulations of small and segmented organisms, rather than to have been derived from the reef framework; there are also indications that at least some of the peloidal sediments were precipitated.The compositions and geometries of the reef mounds at Lime Peak are inferred to have been related to local tectonic events as follows: thick mounds composed mainly of lime sand and mud with only small patches of framework formed during periods of relatively rapid subsidence, whereas thin, tabular mounds composed mainly of framework with aprons of lime sand and mud formed during periods of relative tectonic quiescence.The isolation of the carbonate complex at Lime Peak and the superimposed nature of the reef mounds within the complex contrast with the thick, laterally extensive carbonate platforms and more widely separated reefs that developed along the margin of the rifting Tethyan Ocean. These differences between the general patterns of carbonate sedimentation in the convergent and rifting margins are thought to reflect the less continuous subsidence and more frequent periods of clastic sedimentation in the convergent margin. However, despite the differences in styles of sedimentation, some of the individual reef mounds at Lime Peak are remarkably similar to some of the Tethyan reefs.