[Olsen, P. E., Fairfield, H. M., and Hemming, S., 1999, Stratigraphic and Geo-
chemical Evidence of the Past Distribution of CAMP Basalts. . Eos, Trans-
actions, American Geophysical Union, Supplement, v. 80, no. 17, p. S318.]

T21C-08 1030h  INVITED

Stratigraphic and Geochemical Evidence of the Past Distribution
of CAMP Basalts

Paul E. Olsen1 (914 365 8491; polsen@ldeo.columbia.edu)
Hanna M. Fairfield2 (hmf@columbia.edu)
Sidney Hemming1 (Sidney@ldeo.columbia.edu)
1LDEO, Palisades, NY 10964, United States
2Columbia Univ., 304 Low, 535W 116th St, New York, NY 10027, United States

McHone (1996) proposes that the geographic distribution of 201 Ma central
Atlantic margin continental flood basalt province (CAMP) dikes reflects the
original distribution of CAMP flows. The present distribution of CAMP flows
is indeed diminished from their original extent by erosion, but their original
size is unknown. The original extent of the flows is important because it
constrains interpretations of the climatic and biotic consequences of this Large
Igneous Province, notably the Triassic-Jurassic boundary (Cordillot, 1996).
Methods constraining the former extent of the flows include: determining the
volume of intact basaltic detritus within Jurassic, syn- or post-flow units; and
estimation of eroded volumes by mass-balance models of geochemical tracers
of the flows in post-flow deposits. Basalt does comprise a significant fraction
of the coarse debris in some units adjacent to bounding faults in major exposed
rifts in eastern North America. However, as a fraction of total Early Jurassic
strata or intact flows, basalt debris is clearly very minor, but it is possible that
basalt debris would be very quickly weathered to clay in the tropical depositional
climate of these basins. The geochemical signatures of basaltic input into post-
CAMP deposits should be recognizable because the pre-rift basement is mostly
non-basaltic in composition. Sediments derived from the pre-rift section should
differ from those derived from the degradation of the CAMP basalts in their Nd,
Sm/Nd, and Sr isotope ratios and rare earth element proportions. A preliminary
assessment of a suite of syn-rift Hartford basin mudstones, from all of the
exposed formations, shows no overall trend in the epsilon Nd or Sm/Nd isotopic
ratios from pre-flow through synflow and post-flow units. A simple two end
member mixing model suggests no detectable basaltic input in any of the mud-
stones. This suggests basalt could not be a significant component of Hartford
basin sediments regardless of weathering and a significant area of basalt was not
available for erosion. This work is preliminary, and tests of the robustness of the
basaltic signature in rift strata known to contain a large weathered basalt fraction
are underway. Fundamental problems with the assumed relationship between the
basins and surrounding areas leading to a gross underestimation of CAMP flows
based on syn-rift strata may include: 1) exposed basins are mostly on the western-
most edge of the rift system, perhaps dominated by a sediment supply from west
of the CAMP dikes; and 2) perhaps the entire central rift system subsided as did
the basins themselves, so that the extra-basinal basalts were not available for
erosion until post-rift inversion. Both appear plausible and suggest the basalt
signatures should be found in early post-rift deposits of the Atlantic Margin of
late Early though Late Jurassic age deposited during tectonic inversion (Withjack
et al., 1998). There is an important distinction between the input of continental
basaltic material by continental weathering and seafloor basalt-seawater interactions.
The effect of the former on geochemical proxies of water-basalt interaction could
significantly post-date the climatic or biotic effects of the eruption, while the latter
would be closer in time to the eruptions themselves. Perhaps the low in the curve
of marine 87Sr/86Sr (Howarth and McArthur, 1997) reflects an early-post rift
input of basaltic material and indicates the time interval that should be examined.
Courtillot, V. et al., 1996, GSA Spec. Pap. 307: 513-525; Howarth, R. J. and
McArthur, J. M., 1997, Jour. Geol. 105: 441-456; McHone, J. G., 1996, Geology,
24: 319-322; Withjack, M. O., Schlische, R. W., and Olsen, P. E., 1998, Am. Assoc.
Pet. Geol. Bull. 82: 817-835.