ORBITALLY-PACED, LATEST TRIASSIC-EARLY JURASSIC MEROMICTIC LAKE DEPOSITS OF EASTERN NORTH AMERICA – THEIR SUNSPOT-MODULATED VARVES, TURBIDITES, AND SUPERERUPTION ASHES

Strata interbedded between U-Pb-dated lavas of the Central Atlantic Magmatic Province (CAMP) (1) in eastern North America are comprised of orbitally-paced latest Triassic and Early Jurassic lacustrine cycles (2). Pacing of these cycles is dominated by climatic precession and less so by obliquity and are modulated by the various eccentricity and inclination cycles. The deep-water phases of climatic precession cycles deposited during times of highest eccentricity have whole-fish-bearing laminites made up of organic-rich/organic poor to carbonate rich/organic rich couplets parsimoniously interpreted as varves formed in paleo-tropical meromictic (perennially, chemically stratified) lakes (3). Still only superficially studied, these yearly cycles are apparently modulated by the ~11-year Schwabe sunspot 6cycle that, like the precessionally-paced cycles and the varves themselves, are not only tracible at the basin-scale, but also between the Newark and Hartford basins at the scale of hundreds of kilometers. Lacustrine turbidites are present, that in at least one cycle in the Hartford Basin, can be traced conservatively over a minimum of 170 km2. Within the deep-water phase of the same cycle, in both the Newark and Hartford Basins, are two thin ashes, ~5 and ~1 mm, unambiguously recognizable between basins over a distance of more than 200 km. Without measurable change in thickness or grain size they suggest Yellowstone-scale supereruptions and possible associated volcanic winters, but of basaltic to andesitic composition, part of the CAMP. All these phenomena recall Anderson’s and Dean’s works on varved deposits (4) including, among others, the Permian Castille evaporites (5), Quaternary Elk Lake deposits (6), and Santa Barbara Basin sediments (7, 8), that along with many of their other works, demonstrated the remarkable richness preserved within these annually-resolved geological archives.

1) Blackburn et al. (2013) Science 340:941. 2) Olsen et al. (2019) PNAS 116:10664. 3) Olsen & Kent (1996) PPP 122:1. 4) Anderson & Dean (1988) PPP 62:215. 5) Anderson (2011) Climate of the Past 7:757. 6) Dean et al. (2002) J Paleolimnol 27:287. 7) Dean et al. (2015) Paleoceanography 30, 1373. 8) Kennett et al. (2017) GSA Abst Prog 49:10.1130/abs/2017AM-306169.