Abstract
This paper describes a ∼200 m-thick section of the Pisco Formation exposed at Cerro Colorado, an important fossiliferous site in the Ica desert. In order to properly place the fauna in its correct relative position, this study establishes the stratigraphic framework within which the different fossil-bearing intervals of this site can be compared and may prove invaluable in future high-resolution studies on the faunal change. Most of the Pisco Formation deposits exposed at Cerro Colorado consist of gently dipping fine-grained sandstones, diatomaceous siltstones and diatomites with minor ash layers and dolomites deposited within nearshore and offshore settings. To facilitate detailed stratigraphic correlations within the Pisco strata for a 30 km2 area, eight marker beds have been defined and large-scale (1:10,000 scale) geological mapping conducted to determine fault positions, styles and offsets. The geological map shows that there are two important angular unconformities in the study area. The first one is the interformational basal unconformity of the Pisco Formation against folded, faulted, and planated Oligo-Miocene rocks of the Chilcatay Formation. The second is a low-angle intraformational erosional discontinuity of up to 4° angular discordance that allows the subdivision of the Pisco stratigraphy exposed in the study area into two informal allomembers. Dating of the exposed succession by diatom biostratigraphy suggests that the age of the lower allomember is late Miocene, whereas the upper allomember is late Miocene or younger.
Highlights
The late Oligocene to middle Miocene Chilcatay Formation and the late Miocene to Pliocene Pisco Formation of the Pisco basin, southern Peru, are characterized by an extraordinarily diverse vertebrate fossil fauna, including toothed whales, baleen whales, sea turtles, and fishes
This paper describes a 200 m-thick section of the Pisco Formation exposed at Cerro Colorado, an important fossiliferous site in the Ica desert
Dating of the exposed succession by diatom biostratigraphy suggests that the age of the lower allomember is late Miocene, whereas the upper allomember is late Miocene or younger
Summary
The late Oligocene to middle Miocene Chilcatay Formation and the late Miocene to Pliocene Pisco Formation of the Pisco basin, southern Peru, are characterized by an extraordinarily diverse vertebrate fossil fauna, including toothed whales, baleen whales, sea turtles, and fishes. In the Ica desert, the Pisco Formation crops out as a series of small (up to 200 m high), rounded hills called ‘Cerros’, most of which represent major fossil-bearing localities At these sites, due to the arid climate and young uplift and exhumation, the stratigraphic section can be traced for several kilometres laterally, providing a unique opportunity to study how faunal assemblages changed through time, whether these changes coincided with palaeoenvironmental shifts, and what their nature might be. Due to the arid climate and young uplift and exhumation, the stratigraphic section can be traced for several kilometres laterally, providing a unique opportunity to study how faunal assemblages changed through time, whether these changes coincided with palaeoenvironmental shifts, and what their nature might be This type of study, requires strong spatial and temporal control over the stratigraphic succession, which can only be provided by the integration of refined stratigraphic data from section measuring, detailed geologic and structural field mapping, geochronological dating, and biostratigraphy. In a companion work to this study, Bianucci et al (in press) establish the lateral and vertical distribution of fossil specimens within the same study area and provide some preliminary insights into the faunal changes that have occurred
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