In Geological Society of America, Abstracts with Programs, vol. 35, no. 3, p. 89.
THE INITIAL CAMP FLOOD BASALT EVENT AND ITS RELATIONSHIP TO OVERLYING CARBONATE-RICH
SEQUENCES (NORTH AMERICA AND NORTHWEST AFRICA): TWO OPTIONS RELATED TO THE
TRIASSIC-JURASSIC BOUNDARY
OLSEN, Paul E., Lamont-Doherty Earth Observatory, Columbia Univ, 61
RT 9W, Palisades, NY 10964-1000, polsen@ldeo.columbia.edu, ET-TOUHAMI, Mohammed,
LGVBS, Département des Sciences de la Terre, Université Mohamed
Premier, Oujda, Oujda, 60 000, Morocco, and WHITESIDE, Jessica H., Lamont-Doherty
Earth Observatory, Columbia Univ, 61 RT 9W, Palisades, NY 10964-1000
Throughout the exposed Triassic-Jurassic basins of Eastern North America
and Morocco the oldest basalt flows are of HTQ composition and lie above
the palynological Triassic-Jurassic boundary. In this same vast region, sedimentary
strata above these flows contain several limestone-rich intervals. In eastern
North America, where there are thick sequences of Early Jurassic strata above
the initial basalts, these limestone sequences are lacustrine and stand out
as extremely unusual in an otherwise siliclastic-dominated system. While
these limestones tend to make up the deeper water phases of Milankovitch
cycles (i.e. division 2 of Van Houten cycles) as is usual for the North American
basins, cycles higher above the initial basalts tend to much less calcareous.
In at least eastern Morocco, the limestones are marine containing substantial
mollusk assemblages.
Another characteristic of the Jurassic strata above and immediately below
the initial basalts is the presence of relatively abundant remains of the
dipteraceous fern Clathropteris, and at least one locality with abundant
Cladophlebis. Ferns are otherwise rare in younger Jurassic strata of eastern
North America. Interestingly, spores produced by Clathropteris comprise the
dominant taxon in the “fern spike” recently shown to carry a modest Ir anomaly
at the Triassic-Jurassic boundary itself.
We propose two different scenarios for the the limestone beds, especially
those of lacustrine origin and the associated abundant ferns. 1) The limestone
deposition and the ferns are responding to a super-greenhouse effect caused
by a asteroid or comet impact at the Triassic Jurassic boundary, or the massive
basaltic eruptions themselves, with effects tapering off over some hundreds
of thousands of years. It may be that the ferns are abundant because of a
high fire reoccurrence rate caused by the super-greenhouse effect. 2) The
limestones are weathering products of vast drainage areas newly floored by
relatively Ca-rich basalt, and the ferns are a consequence of unusually heterogeneous
depositional environments caused by the accelerated tilting and subsidence
associated with the eruption of the basalts. These scenarios are not mutually
incompatible, but they do predict completely different far field effects
that can be looked for in other regions far from the CAMP.