The Penas Altas Formation, deposited in a shallow-marine shelf environment, forms the lower part of a Cretaceous transgressive sequence. Starting with a basal conglomeratic clastic wedge, the formation passes upward into a shelf facies and culminates in the basinal Luna Formation. The important carbonate lithofacies include (1) algal wackestones with trace fossils of Planolites,

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Chondrites and Thalassinoides, (2) glauconitic wackestones, (3) rudist-Orbitolina wackestones, and (4) oolitic-bioclastic grainstones. These carbonate facies are associated with cross-bedded coastal marine and platform quartzarenites and silty carbonaceous lagoonal facies. Facies 1, which overlies the basal transgressive clastic wedge, is interbedded with thin rudist-bearing wackestones and algal grainstones. This algal wackestone facies represents deposition in restricted marine back-bar/back-reef conditions. The glauconitic facies 2 was also deposited in a relatively tranquil environment of the back-bar. Facies 3, of alternating thin Orbitolina and rudist-wackestone, contains numerous biogenic tubular structures filled with Orbitolina grainstone. The rudists (caprinids and Toucasia) are mostly unoriented and matrix-supported. This facies probably represents local reefal-biostromal development on an oxygenated marine shelf. The oolitic grainstone (facies 4) is a facies diagnostic of shoaling-upward sequences within or at the margin of the platform. Ammonites and ahermatypic solitary corals at the top indicate a gradual deepening of the environment. The overlying basinal Luna Formation represents the acme of transgression of the Cretaceous period which resulted in an overall upward-fining megacyclothemic sequence.

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