17 Eastern North American Rift Valleys Through Time

Monday, 21 March 2016
1:30 PM - 5:30 PM
Meeting Room 3

WILD AND WONDERFUL IMPLICATIONS OF THE 5 MM POMPTON ASH OF THE HARTFORD AND NEWARK BASINS (EARLY JURASSIC, EASTERN NORTH AMERICA)

Paper number: 17-8
Program: Theme Sessions
Time: 3:30 PM – 3:50 PM

Authors

Abstract

Airfall ashes or tuffs are extraordinarily rare in Triassic-Jurassic rift basin deposits. The Pompton Ash, named for an outcrop in Pompton, NJ, is an exception. In addition to its type locality, it is recognized in two cores in the Towaco Fm. of the Newark Basin and two cores and five outcrops in the East Berlin Fm. of the Hartford Basin. The graded, apparently andesitic ash consists of euhedral, non-rounded, plagioclase laths in clay or chalcedony matrix that was originally glass, fine-grained feathery feldspars, carbonate, and distinct sub-mm spherule-like volcanic grains at the base. Pyrite is abundant, and the ash weathers to an expanded bright orange jarositic mush. Several aspects of this ash are surprising. First, its thickness does not change over its 10 known sites and 200+ km extent, implying it is either the product of a huge, distant eruption, or a smaller eruption closer and positioned just-so. Second, there is a <1 mm ash a few cm higher that also does not change thickness. Third, while validating that ~30-year-old lake cycle correlations between basins are shown correct by the presence of these ashes, it is astounding that these ashes are enclosed by congruent patterns of microlaminae over the same distances. This implies either that the Newark and Hartford basins were connected in a single giant rift lake, or that seasonal to centennial climatic variations overrode all other sources of sediment variability, or both. It is noteworthy that no such correlation has been described for separate extant lakes. Fourth, as has been seen in some basaltic ashes, the Pompton Ash has a modest Ir anomaly, suggesting similar ashes might be the source of more cryptic Ir anomalies in other Triassic-Jurassic strata. Fifth, not only can we evidently correlate strata among various Triassic-Jurassic basins at the 20 ky cycle level, for some intervals we can confidently also correlate at the seasonal scale. Sixth, trace element chemistry, particularly Sm-Nd, should enable recognition of eruptive products in Newark Supergroup and related strata around the end-Triassic extinction where pedogenic processes and chemical weathering may have obliterated textural and mineralogical evidence of ash. Finally, while it is tempting to ascribe these ashes to the Central Atlantic Magmatic Province (CAMP), that conclusion is not yet compelling.

Geological Society of America Abstracts with Programs. Vol. 48, No. 1

doi: 10.1130/abs/2016NE-272509

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