Abstract--Seismic lines, inferred cross-sections, and outcrop studies
reveal the progressive onlap of rift basin strata onto the hanging wall
"basement" block in both surbmerged and exposed Early Mesozoic extensional
basins of the Eastern U.S. as well as the Railroad Valley graben of the
Basin and Range. These relationships indicate that the depositional surface
area of the basins was growing in size through time. Based on these constraints,
we have constructed semi-quantitative models for the stratigraphic evolution
of extensional basins, assuming constant-volume input of sediments and
water and a uniform subsidence rate. When parameterized for the basins
of eastern North America, the models correctly predict a switchover from
fluvial to lacustrine deposition (reflecting the point in time prior to
which a given volume of sediment exceeded the capacity of the growing basin
and after which that the same volume of sediment no longer could fill the
basin, the excess capacity of which was now filled with a lake) and the
gradual exponential decrease in sedimentation rate (thickness divided by
time) after the onset of lacustrine deposition, reflecting the fact that
the sediments and water were spread over a larger and larger area as the
basin grew in size. The close accord between the model's predictions and
the observed stratigraphic record (in which sedimentation rates are calibrated
by 21,000-year-Milankovitch lacustrine cycles) for most of the history
of the basins suggests that the assumptions of the model are not violated.
Deviations occurred in the Early Jurassic, when increased extension led
to an increased basin asymmetry in these half-graben, causing sediment
and water to shift toward the border fault side of the basins, thereby
markedly increasing sedimentation rates in that region of the basin. These
models allow us to extract from the sedimentary record those events in
the history of an extensional basin which are due solely to the filling
of a basin which is growing in size through time (the model's predictions)
and those which are due to changes in tectonics, climate, or sediment and
water budgets (the deviations from the model). The models also indicate
that the present (2-D) models of rift basin evolution (domino-style faulting
and hanging wall collapse) are inadequate to produce basins in which the
depositional surface area must grow in size through time and produce the
stratigraphic sequences observed in these basins.