Ashton F. Embry1
(1) Geological Survey of Canada, Calgary, AB
Abstract: Global tectonics in the Triassic
Stratigraphic data for the Triassic from North America, western Europe and Asia indicate that 12 high order Triassic sequence boundaries can be considered global in extent. The ages of these boundaries are (1) near Permian-Triassic boundary, (2) late Dienerian, (3) late Smithian, (4) near Early-Middle Triassic boundary, (5) late Anisian, (6) near Middle-Late Triassic boundary, (7) early Carnian, (8) mid-Carnian, (9) near Carnian-Norian boundary, (10) mid-Norian, (11) near Norian-Rhaetian boundary, (12) near Triassic-Jurassic boundary.
An examination of the attributes and changes across these boundaries indicate that they were in part generated by tectonics. Evidence for this includes (a) documentation of tilting of beds, (b) major changes in the sedimentary regime including source areas across the boundaries, (c) changes in subsidence and uplift patterns across the boundaries, (d) volcanism at the boundaries.
With these data, the interpretation that global boundaries signify eustasy and regional ones signify tectonics must be abandoned. Tectonics, like eustasy, can have a global effect.
To explain global tectonics, it is proposed that the sequence boundaries are a consequence of episodic, major plate tectonic reorganizations. During these episodes, changes in spreading rates and/or directions would induce changes in the horizontal stress regimes of both the oceanic and continental portions of each plate. This would result in both oceanic subsidence (eustatic fall) and tectonic uplifts along basin margins. These effects would create a widespread unconformity (sequence boundary) in basins on all continents. During the subsequent relaxation phase both ocean uplift (eustatic rise) and tectonic subsidence on basin margins would occur, initiating a new sequence.
It is likely that Triassic sequence boundaries are not unique in being generated by global tectonics. Such a mechanism probably has operated throughout the Phanerozoic.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana