Depositional environments and landscapes of the upper Miocene Ipururo Formation at Shumanza, Subandean Zone, northern Peru

Abstract

During the late Miocene, the Andean–Amazonian region experienced drastic climatic and environmental changes, notably due to a major phase in the Andean uplift. The fossil record is virtually undocumented for this period in the Subandean Zone, where very few palaeoenvironmental and palaeontological investigations have been undertaken. Here, we describe plant remains (pollen, spores, and leaves), microfossils, mollusks, and vertebrates from the Ipururo Formation at Shumanza, San Martín, Peru. Twenty-nine plant families are identified from 164 pollen grains and 89 spores, among them Lycophytes, Monilophytes, and angiosperms (5 monocots and 18 eudicots). The pollen sample notably includes Grimsdalea magnaclavata, Palaeosantalaceaepites cingulatus, Echitricolporites spinosus, and Fenestrites longispinosus, pointing to a late Miocene–early Pliocene age for the TAR-27 locality (10.06–3.72 Ma). Leaf impressions, from nearby localities in the same section, document Malvaciphyllum sp. (Malvaceae), three morphs resembling Caryocaraceae, Fabaceae, Myrtaceae, and two unidentified ‘Dicotyledonae’ angiosperms. The mollusk assemblage is somewhat reminiscent of early–middle Miocene Pebasian faunas and dominated by gastropods (ampullariids, cochliopid, cerithioid, and planorbids). It also includes sphaeriid and unionoid bivalves. Vertebrate recovery is very poor, with a serrasalmine characiform and unidentified actinopterygian teeth. Fossil assemblages and sedimentary facies consistently testify to the dominance of riverine/alluvial forests and the persistence of a steady lowland rainforest close to the Andes less than 10.1 million years ago, without indication of (1) mangrove/marine environments or (2) high-elevation ranges in the close surroundings of Shumanza by that time. By coupling palynostratigraphy and lithostratigraphy, Shumanza fossil assemblages would be further assigned an early late Miocene age (10.1–ca. 8 Ma).