Coniacian-Santonian organic-rich Loma Chumico Formation of N-Costa Rica: An expression of Oceanic Anoxic Event 3?

Abstract

Introduction The Loma Chumico Formation (LCF) of N-Costa Rica (Fig.1), long known for its hydrocarbon potential (Erlich et al. 1996), has been considered by many as of Albian age, based on an single Ammonite determination (Az.ma et al. 1979). Hence, this formation has been considered as a prime source rock formed during OAE 1 (Astorga 1987). Later, first radiolarian dating (Erlich et al. 1996) suggested a younger, poorly defined Cenomanian to Campanian age for this formation, more compatible with the age of the underlying basement. Our systematic radiolarian research in both organic-rich shales (Loma Chumico Facies) and volcanogenic green chert and siliceous mudstones (Berrugate Facies, Andjic et al. 2018) revealed identical, well-preserved radiolarian assemblages (fig.2) of Coniacian-Santonian age for the lower part of the formation. This age is in agreement with Ar/Ar ages of igneous rocks measured in the underlying basement re-defined by us as the Manzanillo Terrane (Andjic et al. 2018). Geologic and plate tectonic setting The Nicoya Peninsula in N- Costa Rica exposes part of a complex puzzle of terranes that are set between the Mesquito Composite Oceanic Terrane (Baumgartner et al. 2008) to the N and the main body of the Caribbean Large Igneous Province (CLIP) to the S (Fig. 1b). Until now, the basements of the whole Nicoya Peninsula were comprised with the Nicoya Complex (Denyer & Baumgartner 2006, Denyer et al. 2014). In our recent work (Bandini et al. 2008) we proposed a subdivision into different terranes which has been consolidated in Andjic et al. (2018). The Nicoya Complex s. str. (sensu Bandini et al. 2008) comprises only the NW-part of the Nicoya Peninsula (Fig 1 c). It is characterized by multiple plateau basalt and intrusive events that can be grouped into three phases of plateau volcanism (Ar/Ar ages: 139—132 Ma, 111 Ma and 95—83 Ma). The youngest phase only (95—83 Ma) is related to the CLIP main pulse (Sinton et al. 1997; Hoernle et al. 2004). 100 m to km sized blocks of ribbon-bedded radiolarite are incorporated into the igneous rocks, and chilled margins indicate generally an older age of radiolarites with respect to encasing igneous rocks. Radiolarian biochronology indicates ages ranging from Bajocian/Bathonian (.168 Ma) to Coniacian-Santonian (.85 Ma, Baumgartner 1984, Denyer & Baumgartner 2006, Baumgartner et al. 2008). No pre-Campanian arc-derived detrital material has been observed in this unit. Unconformable, middle Campanian overlap shallow water and off-shore carbonates encroach directly on the Nicoya Complex s. str. The Manzanillo Terrane covers the major, SE portion of the Nicoya Peninsula (Fig 1 c) It is composed of plateau igneous rocks (120—116 and 96—90 Ma; Sinton et al. 1997; Hoernle et al. 2004; Madrigal et al. 2016) intruded by an OIB-like alkaline suite (~ 89 Ma, Tortugal; Alvarado et al. 1997). This terrane is related to the main pulse of the CLIP (95—89 Ma), it cannot be distinguished by geochemistry from the Nicoya Complex s. str., but it has a shorter age range and does not contain older than Coniacian sedimentary rocks. Its overlap formations consist of the Coniacian to Campanian Loma Chumico, Sabana Grande, and Namb. formations (Bandini et al., 2008). The Loma Chumico Formation (LCF) directly encroaches on the Manzanillo basement and consists: 1) of arc-derived tuffaceous turbidites, debris flows and interlayered green tuffaceous radiolarian-rich cherts (Berrugate facies) and is interstratified with 2) organic-rich siliceous shales (Loma Chumico facies; Astorga 1987, 1997; Erlich et al. 1996; Denyer et al., 2014). The "Berrugate Arc" was located east of the present day Nicoya Gulf, and preceded the Late Campanian initiation of the Mid-American Arc (Buchs et al. 2010). During the middle Campanian the N-Nicoya-Tempisque area was affected by tectonic uplift (Baumgartner et al. 1984) related to the accretion of the Nicoya Complex, resulting in nearshore and shelf deposits, while S-Nicoya (Manzanillo Terrane) remained at pelagic depths and was the site of well-oxygenated limestone deposition (CORBs, Piedras Blancas Formation; Flores et al. 2003, base dated as late Campanian by the presence Radotruncana calcarata). Hence, the Loma Chumico organic-rich facies and the Berrugate arc deposits are restricted to a Coniacian to early late Campanian age. Discussion Erlich et al. (1996) described a total thickness of all organic-rich shales in the Morote-1 core as >85 m with an average TOC of 15.5 %. They characterized the kerogen of the Loma Chumico Formation as half of Type I-II (algal/bacterian/zooplankton) and half of Type III-IV (oxidized/inert) The abundance of the latter may be related to a high heat flow form the fresh, underlying basement. Black shale in direct contact with the basalts of the basement has a highly lustrous aspect suggesting high vitrinite content. A high algal component deduced from their biomarker analyses may reflect to us the presence of algal symbionts of radiolarians, dominant in the fossil assemblages, as well as silicoflagellates common in modern upwelling areas, but rarely preserved in the fossil record. The recovery of abundant ash and tuff layers in the LCF were interpreted by Erlich et al. (1996) as the main source of dissolved silica in the water column and hence, the main enhancer of biological productivity and organic matter burial. Our observations in thin sections of the LCF rather indicate that frequent ash falls resulted in tuffaceous layers that show a lower abundance of radiolarians than the cherts, suggesting a decline of zooplankton production, but a better radiolarian preservation, while the bedded green cherts represent ash-poor, higher productivity episodes. The preservation of organic matter could have been favored by rapid burial under ash-rich layers in an overall dysoxic or anoxic silled basin environment attested by laminated, non-bioturbated shales (Andjic et al. 2018). Erlich et al. (1996) conclude from distinct layers rich in derivates of volcanic glass, episodic high volcanogenic silica input and hence radiolarian blooms. Conversely, we observe excellent biogenic opal preservation of radiolarian tests in ash-rich layers due to a high concentration of volcanogenic silica, that must have protected the radiolarians from diagenetic transformations. Planktonic Foraminifera, and carbonate in general, are rare in the LCF. The first well preserved planktonic foraminifera occur in the overlying, early late Campanian Piedras Blancas Formation. High carbonic acid concentrations in an oxygen minimum zone of the Tempisque silled basin may have resulted in almost total dissolution of calcareous plankton during its descent in the water column, while radiolarians were relatively protected in the acidic environment. Erlich et al. (1996) related the formation of organic-rich layers in the LCF to the existence of the Eastern Pacific Tropical Upwelling Zone (e.g. Sliter & Premoli-Silva 1990), while the renewed oxidation since the early Late Campanian is ascribed to the sudden influx of oxigenated deep water of Antarctic origin through the proto-Caribbean straits. The revised Coniacian-Santonian age of the LCF corresponds with a widespread coeval occurrence in the Caribbean Plate of both red ribbon radiolarites as the earliest sediment on the CLIP, and greenish tuffaceous radiolarian cherts as arc-related hemipelagic siliceous sediments. Both abundant radiolarian and organic matter productivity/preservation may be related with the Oceanic Anoxic Event 3 (Jenkyns 1980) of Coniacian-Santonian age, so far mainly represented by a widespread and stratigraphically long black shale record in the Atlantic region (Jenkyns 2010). OAE 3 is characterized by modest positive shifts of .13C recorded in the English chalk reference curve and the Gubbio sections by Jarvis et al. (2006), as the White Fall, Kingsdown, Horseshoe Bay and Haws Brow events. This correlation downscales the hydrocarbon potential and the regional importance of the LCF.

AAPG Datapages/Search and Discovery Article #90330©2018 AAPG Hedberg: Geology of Middle America – the Gulf of Mexico, Yucatan, Caribbean, Grenada and Tobago Basins and Their Margins “Integration of the geology of the area from the southern onshore of North America to the northern onshore of South America”, Siguenza, Guadalajara, Spain, July 2-5, 2018