Tectonostratigraphic development of Mesozoic rift
basins and the evolution of eastern North America
Abstract--Studies of outcrops, drill cores, and seismic reflection
profiles supplemented by subsidence analysis and forward basin filling
modeling of exposed and buried early Mesozoic rift basins suggest the following
tectonostratigraphic history for eastern North America: (1) Basin subsidence,
controlled largely by the reactivation of Paleozoic faults, began in the
early to middle Carnian (Triassic). In all of the exposed basins, initial
sedimentation was fluvial, reflecting an oversupply of sediment with respect
to the initially small capacities of the growing basins. (2) Fluvial sedimentation
gave way to lacustrine sedimentation as the increasing capacities of the
basins eventually overwhelmed the sediment supply; the fluvial-lacustrine
transition occurred at different times in different basins, suggesting
that it was not controlled by a regional tectonic event. (3) Decreasing
accumulation rates in Late Triassic lacustrine strata may indicate that
basin capacity increased faster than sediment supply rates. (4) Subsidence
slowed toward the Triassic-Jurassic boundary in the northern basins, permitting
them to fill to their lowest outlets, allowing fluvial deposits to accumulate
throughout many of the basins. Subsidence stopped completely in the southern
basins. (5) Extension and subsidence rates markedly increased shortly before
the Triassic-Jurassic boundary, creating deep, narrow depositional basins
in which thick, deep-water lacustrine deposits accumulated. In the earliest
Jurassic (201 Ma), quartz-normative, tholeiitic flood basalts and associated
intrusives were emplaced over a period of only 650 kyr; shortly thereafter,
subsidence and extension rates slowed considerably, and at least one basin
eventually filled to its lowest outlet. (6) A second thermal event, manifested
by hydrothermal alteration (causing the remagnetization of rocks and resetting
of isotopic clocks) and the emplacement of seaward-dipping extrusives at
the continent-ocean boundary, heralded the initiation of seafloor spreading.
(7) The region landward of the hinge zone, which was likely still standing
above sea level (as indicated by the exlusively terrestrial deposits of
the exposed rift basins), was then eroded. Isostatic uplift and erosion
combined to remove up to 5 km of synrift strata from some of the rift basins.
(8) Flexural loading of the passive margin eventually caused the rift basins
to subside below sea level, allowing marine post-rift strata to unconformably
overlap the synrift deposits.