Palaeoceanography of organic-rich rocks of the Loma Chumico Formation of Costa Rica, Late Cretaceous, eastern Pacific



The Late Cretaceous (late Cenomanian-Campanian) Loma Chumico Formation consists of laminated to massive bedded siliceous shales, cherts, ash fall tuffs/volcanic glass beds and volcaniclastic sandstones. The Loma Chumico Formation is at least 630 m thick (core data) in the Tempisque Basin, and can be subdivided in the subsurface into organic-rich and organic-lean (total organic carbon <1%) facies. Rock-Eval total organic carbon (TOC) data show that TOC in the organic-rich facies averages 15·5%, with values as high as 33%. Inorganic geochemical, petrological and sedimentological analyses indicate that all the lithologies of the Loma Chumico Formation were deposited in middle shelf to outer slope environments adjacent to an active subaerial island arc.

The organic geochemistry and palaeontology of the Loma Chumico Formation help constrain the factors that controlled biological productivity in the region during the Late Cretaceous. Biomarkers, identified in organic-rich facies by gas chromatography/mass spectrometry (GC/MS), show a high abundance of total steranes relative to hopanes in the Morote-1 core, indicating algal-dominated organic input. The hopane-to-sterane ratio of the extract from the Manzanillo-1 core indicates a relatively high bacterial component to the organic matter.

Upwelling and extrusive volcanism were the most important factors that controlled nutrient supply (including silica flux) and biological productivity within the Loma Chumico Formation depositional system. Episodic radiolarian, dinoflagellate or diatom ‘blooms’, instigated by rapid infusions of volcanogenic silica, may have resulted in expansion of the oxygen minimum zone (OMZ), enhancing the preservation of organic matter. Productivity appears to have been uniform, even during deposition of the organic-lean facies.

The findings of this study suggest that ocean-wide anoxia in the eastern Pacific was unlikely during the Late Cretaceous. High TOC contents of organic-rich radiolarites from central Caribbean Deep Sea Drilling Project (DSDP) sites may be related to upwelling produced in the eastern Pacific by convergence of the easterly Trade Winds (Eastern Pacific Equatorial Upwelling Zone), or may be part of a more complex Pacific-Atlantic Ocean interaction.

The end of deposition of organic-rich rocks began near the end of the latest Santonian-earliest Campanian in the central Caribbean and Costa Rica.

Palaeoceanographic models suggest that improved circulation and ventilation of bottom waters due to early opening of the North Atlantic-South Atlantic seaway may explain this observation. This may represent the influx of a previously unrecognized source of colder, oxygenated intermediate/bottom water (proto-Antarctic Intermediate Water) into the Late Cretaceous central Atlantic and eastern Pacific oceans.