The exceptional paleontological site in the Peruvian Pisco Basin

by Giulia Bosio |

In the dry and dusty Peruvian desert of the Ica Region (Peru), the sedimentary rocks host an exceptional paleontological record, renowned for its marine vertebrate fossil content. From the ’80s, whales, dolphins, seals, sharks and other fishes, birds and reptiles were discovered and studied by paleontologists, revealing an exceptional Fossil-Lagerstätte, i.e. a sedimentary deposit with abundant and excellently preserved fossils (Bosio et al., 2021).

Fig. 1 – A fossil mysticete discovered in the Ica Desert. Photo by Giovanni Bianucci.

Fig. 2 – Base camp in the Ica Desert in Peru. Photo by Giovanni Bianucci.

Fig. 3 – The East Pisco Basin sedimentary succession exposed in the Ica Desert. Photo by Giulia Bosio.

Marine sediments accumulated in the Pisco Basin from ca. 40 to 5 million years ago (Di Celma et al., 2022). The inhabitants of this ancient sea bay are now exposed to the desert due to tectonic uplift and erosion. In this basin, different paleoenvironments have followed one another in geological time, starting from a tropical shelf setting with a moderate nutrient supply in the middle-late Eocene towards a cooler and nutrient-rich environment in the late Miocene dominated by upwelling conditions and the strengthening of the Humboldt Current (Collareta et al., 2021).

In this context, diatomaceous sediments mainly characterize two formations: the late Eocene to early Oligocene Otuma Formation and, to a greater extent, the middle to late Miocene Pisco Formation. In the field, the outcrops are white to pinkish and comprise finely laminated to massive diatomites. In some cases, diatomaceous siltstones reach tens of meters of thickness, resulting in soft and rounded white hills in the desert that resemble a snowy landscape in the night.

Fig. 4 – Diatomite strata of the Pisco Formation. Photo by Giulia Bosio.

These diatomaceous strata are often alternated with volcanic ash layers that originated from the Central Andes (Bosio et al., 2020). Volcanic material input usually increases nutrient and, consequently, diatom concentrations.

Diatomaceous sediments also host a great abundance of cetacean and other vertebrate fossil specimens, usually finely preserved. The original soupy consistency of the sediment, the suboxic environment at the seafloor, the rapid burial of the carcasses by the scour-induced self-burial process, and the diagenetic precipitation of dolomite around the carcasses helped in preserving not only the bones but also organic matter such as baleen (i.e., a keratinous filter-feeding system inside the mouths of baleen whales similar to bristles) and digestive tract contents (Bosio et al., 2021).

Under the microscope, these whitish fine-grained sediments reveal siliceous plankton remains such as diatoms and silicoflagellates, as well as sponge spicules. Diatom valves are the most abundant and the association is dominated by coastal species. Recognizing the diatom genera and species allows the reconstruction of the paleoecology of the basin and the paleoclimatic changes.

The most common genera in the Otuma Formation are Hemiaulus, Pyxilla, Rhizosolenia, Stephanopyxis and Cestodiscus, indicative of stratified summer waters and related to a seasonally-limited upwelling (Malinverno et al., 2021). In the youngest Pisco Formation, the association is dominated by the high-productivity genera Thalassionema, Chaetoceros and Rhizosolenia, but also Actinoptychus, Coscinodiscus, Delphineis, Denticulopsis, Fragilariopsis, Lithodesmium, Nitzschia, Paralia, Stephanopyxis and Thalassiosira (Gariboldi et al., 2017).

Fig. 5 – Diatom frustules under the microscope from the Pisco Formation. Bar scale: 10 µm.

Diatoms are also helpful in dating the sedimentary rocks of the Pisco Basin. Diatom biostratigraphy is particularly useful for dating Miocene units in the Pisco Formation. For example, species such as Lithodesmium reynoldsii and Denticulopsis praekatayamae are very valuable for dating sediments of ca. 10 and 8.5 million years ago in the Pisco Formation (Gariboldi et al., 2017). Diatom biostratigraphy is also used in the Otuma Formation, for Eocene sediments of ca. 35 million years ago (Malinverno et al., 2021).

Fig. 6 – The diatom species Lithodesmium reynoldsii from the Pisco Formation. Bar scale: 10 µm.

References:

Bosio, G., Gioncada, A., Di Celma, C., Villa, I. M., Pichavant, M., Urbina, M., Bianucci, G., 2020. Two-mica rhyolitic tephra in the East Pisco Basin (Peru): new age and dispersion constraints for the eruptions of the Eastern Cordillera of Central Andes. Bulletin of Volcanology, 82, 1-22.

Bosio, G., Collareta, A., Di Celma, C., Lambert, O., Marx, F.G., de Muizon, C., Gioncada, A., Malinverno, E., Varas Malca, R., Urbina, M., Bianucci, G., 2021. Taphonomy of marine vertebrates of the Pisco Formation (miocene, Peru): insights into the origin of an outstanding fossil-lagerst¨atte. PLoS One 16 (7), e0254395.

Collareta, A., Lambert, O., Marx, F.G., de Muizon, C., Varas-Malca, R., Landini, W., Bosio, G., Malinverno, E., Gariboldi, K., Gioncada, A., Urbina, M., Bianucci, G., 2021. Vertebrate Palaeoecology of the Pisco Formation (miocene, Peru): glimpses into the ancient humboldt Current ecosystem. J. Mar. Sci. Eng. 9, 1188.

Di Celma, C., Pierantoni, P.P., Volatili, T., Molli, G., Mazzoli, S., Sarti, G., Ciattoni, S., Bosio, G., Malinverno, E., Collareta, A., Gariboldi, K., Gioncada, A., Jablonska, D., Landini, W., Urbina, M., Bianucci, G., 2022. Towards deciphering the Cenozoic evolution of the East Pisco basin (southern Peru). J. Maps 18, 397–412.

Gariboldi, K., Bosio, G., Malinverno, E., Gioncada, A., Di Celma, C., Villa, I.M., Urbina, M., Bianucci, G., 2017. Biostratigraphy, geochronology and sedimentation rates of the upper Miocene Pisco Formation at two important marine vertebrate fossil-bearing sites of southern Peru. Newsl. Stratigr. 50, 417–444. https://doi.org/10.1127/nos/2017/0345.

Malinverno, E., Bosio, G., Di Celma, C., Gariboldi, K., Gioncada, A., Pierantoni, P.P., Collareta, A., Molli, G., Bagnoli, G., Sarti, G., Urbina, M., Bianucci, G., 2021. (Bio)stratigraphic overview and paleoclimatic-paleoceanographic implications of the middle-upper Eocene deposits from the Ica River valley (East Pisco basin, Peru). Palaeogeogr. Palaeoclimatol. Palaeoecol. 578, 11056 https://doi.org/10.1016/j.palaeo.2021.110567.