AIR-FALL ASHES OF THE CAMP FROM EASTERN NORTH AMERICA AND MOROCCO

Until recently, there have been no unequivocal distal air-fall ashes or tuff described from the entire extent of the Central Atlantic Magmatic Province (CAMP). However, several have been identified recently and they may be more common than previously thought.

1) The 5 mm Pompton Tuff Bed occurs in a deep-water phase of a lacustrine cycle at 7 localities in the East Berlin and Towaco formations of the Newark and Hartford basins over 230 km. The tuff is comprised of graded sub-mm volcanic fragments containing ~0.1 mm long, high-aspect ratio, euhedral plagioclase laths in an originally glass and fine-grained feather feldspar matrix. Electron microprobe data show the most calcic large plagioclase crystals to be An46 to An22 suggesting an andesite. This tuff appears to be the deposit of a distal ash plume of a large explosive eruption, and demonstrates that the correlations of lake cycles both within and between the Newark and Hartford basins are correct.

2) Thin, graded, volcaniclastic beds below the pillowed Talcott Formation in the Meriden, CT area consists of basalt clasts with plagioclase phenocrysts and altered euhedral olivine crystals in a fine-grained groundmass of feathery looking feldspar. While these layers might be debris from the steep front face of advancing forsets of pillows, the pattern of graded beds is consistent over a distance of at least 1 km suggesting the source of the beds was not immediately local.

3) Volcanic granules in a thin bed below the Talcott Formation in the Silver Ridge B-2 core in Berlin, CT appear to be more silicic than the beds in Meriden with compositions in the An 25-35 range overlapping the range of the Pompton Tuff Bed. The granules also seem to exhibit trachytic or trachytoidal flow.

4) A 2 cm candidate tuff occurs 3 cm below oldest basalts in Morocco, which are arguably the oldest flow sequences known in the CAMP, at the end-Triassic mass extinction level. Pending thin section analysis, the Igounan candidate tuff would predate these lavas and perhaps indicate a yet older phase of CAMP explosive eruptions.

These tuffs are important not only as evidence of explosive CAMP eruptives, but also as markers, elucidating the stratigraphy of one of the largest and most consequential volcanic episodes on Earth.