Lower Cretaceous paleobotany and stratigraphy insights from the Central South Andean zone of Ecuador

Abstract:

Cretaceous paleofloras from South American low latitudes are poorly understood due to scarce research exploration that has been conducted in this area. Today, this region holds high plant species diversity, therefore, investigating Cretaceous macrofloras can shed light into the initial diversification of flowering plants and also to the paleoenvironments in which the early Cretaceous forests grew in Gondwana. Recently, we explored the paleobotanical record and stratigraphy of the Hollin Formation, a clastic stratigraphic unit dated as Aptian to Albian, outcropping in the in Central Eastern Subandean Zone of Ecuador. Aiming to reconstruct the paleoenvironment of the Hollin Formation, we collected and studied rock samples and macrofossil plants, and we described stratigraphic sections in detail. The stratigraphic columns show horizontal-laminated and cross-bedded medium to coarse sandstones forming fluvial channels and fluvial plains, interbedded with few horizontally-laminated shales and siltstones from bay and clastic shelf. Furthermore, into a few of these sequences, there is evidence of amber that proofs the abundant paleoflora of the region. From the macrofossils collected we observed leaf samples with parallel- venation, fronds, and also well-preserved cuticles. This analysis evidences a clastic depositional environment in the eastern SAZ of Ecuador dominated by braided stream rivers, with some influence of fine sediments of oxbow lakes, bay, and brief floodings of a shallow sea. In terms of flora, the possible taxonomic affinities of these fossils suggest the presence of Selaginella, two types of fern fronds, podocarpaceous shoots and leaves, Brachyphyllum, and multiveined leaves of Podozamites. Remarkably, the Hollin flora also preserves abundant compression and leaf cuticles of at least five angiosperms, making this flora one of the earliest records of this group in tropical South America. Our results contribute to our understanding of the evolution of the tropical belt, which is today the most biodiverse region in the world.