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

Abstract

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.

Rapid vertical motions and formation of volcanic arc gaps: Plateau collision recorded in the forearc geological evolution (Costa Rica margin)

“…Extracted radiolarian assemblages indicate that the base of the Loma Chumico Fm. is no older than the Coniacian (89.8-86.3 Ma; Figures 4 and 5), confirming previous Late Cretaceous age assignments based on radiolarians (Erlich et al, 1996;Schmidt-Effing, 1979). Radiolarian assemblages of similar Coniacian and younger age were also recovered from tuffaceous rocks in the Nicoya Gulf (Berrugate Fm.…”

“…The erosion of such reliefs was documented by the deposition of turbidites and boulder-bearing, pebble debris flows (Brecha Puerto Carrillo;Sprechmann, 1984;Baumgartner et al, 1984;Figures 3a and 8a). during "OAE3" (Erlich, Villamil, & Keens-Dumas, 2003). This suggests that there were no sediment-transport pathways between the Tempisque Forearc Basin and the MCOT.…”

“…The oldest overlap formations overlie the Manzanillo Terrane and correspond to the Coniacian-late Campanian turbiditic sequences of the Loma Chumico, Sabana Grande and Namb ı formations . consists mainly of tuffaceous debris flows and turbidites (Berrugate facies), locally interstratified with black shales (Loma Chumico facies; Astorga, 1987Astorga, , 1997Erlich, Astorga, Soler, Patts, & Palmer, 1996;Flores et al, 2003a;Denyer et al, 2014a). The Loma Chumico Fm.…”

“…Moreover, silled basins isolated by submarine highs potentially represented quiet environments which favoured the subsequent deposition of the bituminous Loma Chumico Fm. during "OAE3" (Erlich, Villamil, & Keens-Dumas, 2003). These barriers acted as a shelter from currents and allowed the storage of organic matter under anoxic conditions.…”

The Petroleum System in the Sandino Forearc Basin, Offshore Western Nicaragua

“…Astorga, 1987), which represents an important source rock interval within the forearc basins of western Costa Rica (see below). Age determinations for the Loma Chumico Formation range widely from the Early to Late Cretaceous -Aptian to Early Campanian (Astorga, 1988; or, alternatively, late Cenomanian to Campanian (Erlich et al, 1996). The deposition of organic-rich intervals has been correlated with oceanwide anoxic events during Aptian-Albian and Cenomanian/Turonian (Astorga, 1988(Astorga, , 1990(Astorga, , 1994Astorga et al, 1991).…”