Basin filling, subsidence, and thermal history of
the Norfolk rift basin, mid-Atlantic margin
Abstract--Recent EDGE PROJECT seismic reflection profiles off Virginia
cross the Norfolk Basin, a buried, presumably Triassic-Jurassic half-graben
on the continental shelf. An east-dipping listric border fault (maximum
dip of 35 degrees) that parallels the grain of Appalachian basement structures
froms the western border of the basin. The basin floor truncates pre-Mesozoic
thrust sequences. The synrift basin fill (3.5 km of strata that thicken
toward the border fault) contains two main units. The lower unit consists
of an inferred fluvial sandstone body overlain by an ~35-m-thick coal layer.
On the basis of the seismic facies from exposed basins, the upper unit
displays a tripartite stratigraphy, consisting of a lower sandstone, middle
shale, and an upper sandstone, interpreted to be a fluvial-lacustrine-fluvial
sequence. This tripartite subdivision is controlled by the interplay between
sediment supply rate and tectonically controlled basin capacity, with the
lacustrine strata accumulating in a sediment-starved basin. Onlap of lower
unit strata onto the hanging wall block indicates that the depositional
basin was widening as it filled. Upper unit strata pinch out against lower
unit strata, indicating that the upper unit was deposited in a smaller
but deeper depositional basin. The width of the depositional basin decreased
from 60 km for the lower unit to 20 km for the upper unit, reflecting an
increase in extension rate or a change in direction of strain. The decrease
in width occurred around the Triassic-Jurassic boundary, the same time
as an increase in extension rate in the Culpeper, Newark, Hartford-Deerfield,
and Fundy basins. Backtracking of the subsidence history to the onset of
rifting is calculated given control ages for rifting, breakup, and flexure,
and using a range of published subsidence and denudation rates. Results
yield the amount of basin sediment eroded as well as the elevation and
time of formation of the coal deposits. Thermal history for the Norfolk
Basin is obtained using the reconstructed depth of the coal layer through
time and the temperature necessary for its formation.