REGIONAL AND GLOBAL CLIMATIC IMPLICATIONS OF
HIGH-RESOLUTION ASTRONOMICALLY CALIBRATED
PALEOMAGNETIC POLARITY TIME SCALE FOR THE LATE
TRIASSIC AND EARLY JURASSIC
OLSEN, P. E. , and KENT, D. V., Lamont-Doherty Earth
Observatory of Columbia University, Palisades, NY
10964
Triassic-Jurassic lacustrine and paralic strata preserved in numerous
rift basins and rift-related basins from Svalbard to the Gulf of Mexico
display a hitherto baffling array of humid to arid facies. These
seemingly conflicting associations of facies have prompted several ad
hoc explanations evoking non-zonal climatic processes such as
monsoons, the effects of orography, and global climate change.
Cyclostratigraphic and paleomagnetic analysis of 6700 m of core from
the Newark basin collected by the Newark Basin Coring Project
(NBCP) provided a high-resolution astronomically calibrated
magnetic polarity time scale for the Late Triassic and Early Jurassic.
About 30 million years and 59 polarity intervals are recorded in this
sequence. Paleomagnetic results show that the position of the
paleoequator was in present-day Virginia during the Carnian of the
Late Triassic and that Pangea drifted about 9ƒ north from the Carnian
to the Early Jurassic.
With this time scale and pole position data in hand, the overall
pattern of climate sensitive facies in the Triassic and Early Jurassic is
greatly simplified. There was a seeming symmetrical arrangement of
facies around the paleoequator during Carnian time. Coals and deep-
water lacustrine deposits were produced at the paleoequator (Deep
River, Dan River, Richmond, and Taylorsville basins) while strikingly
cyclical lacustrine and playa deposits and bioturbated red beds were
produced 10ƒ to the north and south. Contemporaneous deposits near
30ƒ N paleolatitude in Greenland and the Haltenbaken area of offshore
Norway consist of eolian sand dunes and evaporite beds, while further
north in Svalbard, deltaic coals and black mudstone again dominate.
This pattern conforms to a simple zonal one, with a narrow equatorial
humid zone, with an arid belts centered on 30ƒ, in turn passing
northward into temperate climates.
To first order, as Pangea drifted northward, the vertical
sequence of climate sensitive facies in individual basins change as
they pass from one climate zone to another. Thus, in the Newark
Supergroup of North America (North Carolina to Nova Scotia) and in
Morocco the transition from Carnian through Norian is characterized
by apparent drying with shallow water cyclical lacustrine strata
predominating in the southern basins (Norian of Newark Gettysburg,
Culpeper, Taylorsville, and Dan River basins). Conversely, in
England, Sweden, Haltenbaken area and Greenland, the Late Triassic
gets progressively wetter with coals being produced by the Rhaetian
in Greenland.
In the Newark Supergroup and Morocco, there is one major,
but short lived, departure from this pattern. In the latest Rhaetian and
earliest Jurassic, more humid cyclical deposits appear simultaneously
with the extrusion of flood basalts. Because this apparent reversal in
the paleoclimatic trend is not accompanied by a drift of Pangea to the
south, it probably reflects a true climate change. This is the kind of
climate change that would be anticipated by the spread of the ocean
closer to the region, as did occur in the Early Jurassic flooding of
western Europe.
On a global scale, most of the apparent large climate changes
seen in early Mesozoic basins can be explained by the northward drift
of Pangea. Thus the drying seen in the classic Colorado Plateau
sequence of the Chinle and Glen Canyon groups in the Western US is
parallel to that seen in eastern North America. Similarly, the drying
seen in the Late Triassic and Early Jurassic sequences of the Karoo
basin and Argentinean and Brazilian basins can be explained by their
drift from the south temperate regions into the southern arid belt.
Thus, in contrast to previous analyses of Triassic paleoclimate,
the overall patterns of Late Triassic- Early Jurassic climate fit into a
simple more or less stationary zonal climate pattern, albeit with a very
narrow equatorial humid zone. These observations emphasize the
need for careful paleogeographic reconstructions constrained by long
sequences of data in direct superposition, before ad hoc, non-zonal
explanations of climate are brought to bear on scattered paleoclimatic
data. Such control is especially important geological data are to be
used in a credible way to validate global climate models designed to
predict future climate.
go back to "MEETING SCHEDULE"