292-5 Quantifying the Campanian Latitudinal Temperature Gradient in the Western Interior Basin using Digital Leaf Physiognomy in the Almond, Neslen, and Kaiparowits Formations

Abstract:

In the Late Cretaceous, the globe was relatively warm and there was a shallow temperature gradient of ~0.3-0.4 ℃/°latitude, compared to ~1.0 ℃/°latitude today. This greenhouse interval was marked by high sea level, creating epicontinental seaways on every major landmass. One such Cretaceous continental seaway was the Western Interior Seaway (WIS) of North America, which connected the Arctic Ocean to the proto-Gulf of Mexico. Within the WIS, cool polar waters mixed with warm southern waters and created a steep temperature gradient within the basin. However, the steepness of the gradient on land is debated, ranging across data sets from 0.6 to 1.5 ℃/°latitude. 

Using Digital Leaf Physiognomy (DiLP), we reconstruct mean annual temperature and rainfall from five sites across three formations within the Western Interior Basin (WIB) – the Almond, Neslen, and Kaiparowits formations – filling a gap in terrestrial temperature reconstruction spanning 47.3-50.9° paleolatitude. 

The Almond Formation crops out in southwestern Wyoming (50.9° paleolatitude) and was deposited as the WIS receded during the Late Cretaceous. We describe a new collection of ~300 leaves represented by 32 morphotypes. We interpret the depositional environment to be estuarine based on the presence of poorly bedded fine sediment, storm deposits, and Teredolites. Previous paleobotanical collections from the Almond Formation were fern and gymnosperm dominated, whereas this site is angiosperm dominated.

The Neslen Formation crops out in eastern Utah in the Book Cliffs (49.4° paleolatitude). These rocks preserve fluvial and nearshore environments from ~75 million years ago. We describe and apply DiLP to two new angiosperm dominated sites, each preserving 150-200 leaves. 

The Kaiparowits Formation in Southern Utah (47.4° paleolatitude) preserves well-studied fluvial strata from the middle Campanian. Fossil plants from two sites, Lost Valley and Caveat Friendship, were previously described. Those sites are diverse and dominated by woody dicots with ~45 morphotypes each, which make them good candidates for DiLP. 

Preliminary results from DiLP indicate that these sites are warm with very little variation in mean annual temperature. Between these three formations, the temperature gradient is similar to average global values.

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