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Abstract:

Triassic sea-level changes are not well documented because of a scarcity of Triassic marine strata over many of the continental interiors and on passive continental margins. An excellent laboratory for studying Triassic sea-level changes is the Sverdrup Basin, which was a major depocenter in the Canadian Arctic Archipelago from the Carboniferous to early Tertiary. Marine Triassic strata are widespread across the basin and are as thick as 4,000 m.

The established stratigraphic pattern for the Triassic succession consists of thick progradational wedges of deltaic and marine strata, alternating with thin, transgressive, clastic units (T-R cycles). On the basin margins, subaerial unconformities cap the progradational wedges, and over much of the basinal area, submarine unconformities form the cycle boundaries. Nine T-R cycles occur in the basin and are interpreted as having been generated by an interplay of eustatic sea-level change, gradually decaying thermal subsidence, and variable rates of sediment supply and load subsidence. In this model, rapid eustatic sea-level rises coincide with major transgressions that occurred in earliest Griesbachian, earliest Smithian, late Smithian, earliest Anisian, early Ladinian, earliest Carnian, mid-Carnian, earliest Norian, earliest Rhaetian, and earliest Jurassic. Progradation occurred in the intervening time intervals under conditions of slow eustatic sea-level rise, stillstand, and fall.

The long duration of each of the sea-level cycles (about 5 million years) and the apparent lack of Triassic glacial deposits indicate the cycles had a tectono-eustatic origin that relates sea-level changes to changes in the volume of the ocean basins. Sea-level rises are related to episodes of increased rates of seafloor spreading and oceanic volcanism that resulted in reduced oceanic-basin volume. The intervals of sea-level fall occurred when seafloor spreading and associated volcanism were subdued and the ocean basins gradually enlarged due to thermal subsidence.

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