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.