UPPER CENOMANIAN SHALLOW-WATER BIVALVES FROM EL SALTO FM ON LA MOLINILLA CREEK (SANTANDER), STRATIGRAPHIC IMPLICATIONS FOR THE SE REGION OF MIDDLE MAGDALENA VALLEY BASIN (COLOMBIA)

El Salto Formation is a conspicuous but poorly studied shallow-water mixed carbonate-clastic unit in the Colombian Middle Magdalena Valley Basin, characterized by extreme regional thickness variations. Here, we present a high-resolution paleontological study of the Oyster-rich shell beds from a seven-meter-thick succession of the El Salto Formation at La Molinilla Creek in the southeast Middle Magdalena Valley Basin, where the formation's thickness is severely reduced. In the Colombian Geological Survey study of the adjacent outcrop, El Salto Formation was dated as Cenomanian based on the acme of Gryphaeidae Rynchostreon squamatum (D'Orbigny, 1842), whereas the underlying siltstones of the Simiti Formation was reported to include lower Upper Albian ammonites, thus indicating an unconformity between the two units.

Our study identified Camponectes cf. striatopunctatus (Roemer, 1839), Ceratostreon cf. flabellatum (Goldfuss, 1833), Amphidonte cf. obliquata (Pulteney, 1813), Oscillolopha syphax (Coquand, 1862), Plicatula auressensis Coquand, 1862 and Pseudoptera cf. gregaria (Shumard, 1860) including the index species of the Middle and Upper Cenomanian Ilymatogyra africana (Lamarck, 1801) which is dominant in packstones beginning at 70cm above the contact with Simiti Fm where the CGS previously identified Rhynchostreon squamatum (D'Orbigny, 1842). However, comparisons of the external morphology and shell microstructure of Ilymatogyra africana with known specimens of Rhynchostreon squamatum collected from the Villa de Leiva region indicate that the latter was misidentified at La Molinilla. Rhynchostreon squamatum was found in very small numbers as juvenile specimens at a single fossiliferous site near La Molinilla Creek, suggesting that such species is rare in the studied area.

Thus, we interpret El Salto Formation as Middle to Upper Cenomanian on La Molinilla Creek based on the reported known vertical distribution and acme of Ilymatogyra africana in the Colombian Upper Magdalena Valley Basin and Eastern Cordillera, as well as Peruvian and North African localities. Our results suggest that the hiatus at the base of El Salto Formation at La Molinilla Creek exceeds the initial estimations by the Colombian Geological Survey.