Nummulitic banks in the upper Lutetian ‘Buil level’, Ainsa Basin, South Central Pyrenean Zone: the impact of internal waves

Abstract

Nummulites, a particularly abundant and diverse genus of larger benthonic foraminifera, formed huge accumulations (banks) during the Eocene, which are often good hydrocarbon reservoirs, especially in North Africa. Despite their economical interest, these accumulations are not well-understood and their origin is still under discussion. Reasons for this debate are the absence of living Nummulites accumulations and the high-variability of facies, including the size, shape and extension of the banks, which reflect the array of processes controlling sediment production and accumulation. The nummulitic banks near Santa María de Buil, in the Ainsa Basin (South Pyrenean Foreland Basin) are composed of recurrent facies associations within mappable bed units bounded by physical surfaces. The depositional processes that produced the Nummulites deshayesi accumulations are interpreted considering the shape of the banks, the type of bounding surfaces, the distribution of sedimentary textures, Nummulites test shapes and the associated skeletal components within the banks. This integrative analysis indicates that nummulitic banks accumulated from mass flows, with very poor sediment sorting. Textural and compositional differences among banks suggest that globose Nummulites thrived in the shallower part of the mesophotic zone with abundant nummulithoclasts, whereas flat nummulitic forms thrived in deeper mesophotic, clay-dominated settings. Interbedded with nummulitic banks, coral biostromes and coral mounds bearing Operculina, Discocyclina and Asterocyclina, represent in situ accumulation near the base of the photic zone. Internal waves (waves that propagate along the pycnocline) are thought to be the triggering mechanism for the density flows. Internal waves and induced bottom currents are sporadic but strong enough to bring sediments into suspension. In contrast to surface waves (both fair-weather and storm), whose impact is strongest near the sea surface and decreases with bathymetry, the impact of internal waves is usually strongest in the mid-shelf region and weaker in shallow water. This explains the compositional character of the nummulitic banks.

Reef building and carbonate production modes in the west-central Tethys during the Cenozoic

“…9) occur within the uppermost composite sequence "Barranco El Solano" of Dreyer et al (1999), overlying deltaic sandstones and clays, in turn overlain by the Bartonian alluvial Escanilla Formation. Coral biostromes, locally biohermal, occur interbedded with the nummulitic lithosomes (Mateu-Vicens et al, 2012). Coral colonies occur sparsely within a packstone matrix, lacking in-situ intergrowths and primary cavities (spaced cluster reef sensu Riding, 2002 In this context, the coral biostromes and coral mounds, bearing Operculina, Discocyclina and Asterocyclina, clearly represent in-situ production and accumulation in meso-oligophotic conditions, below the zone inhabited by Nummulites (Mateu-Vicens et al, 2012).…”

“…Middle Eocene coral buildups occur in the Ainsa fold and thrust Oblique Zone, where a thick progradational deltaic to alluvial succession (Sobrarbe and Escanilla Fms) was deposited between the Mediano, Boltaña and Balces growing anticlines (Poblet et al, 1998;Dreyer et al, 1999;Barnolas et al, 2004;Arbués et al, 2011). Four regressive composite sequences build the Sobrarbe deltaic complex (Dreyer et al, 1999), which includes several thin Nummulites-rich horizons, often with glauconite, and locally with small coral-bearing mounds (Mateu-Vicens et al, 2012).…”

“…Coral biostromes, locally biohermal, occur interbedded with the nummulitic lithosomes (Mateu-Vicens et al, 2012). Coral colonies occur sparsely within a packstone matrix, lacking in-situ intergrowths and primary cavities (spaced cluster reef sensu Riding, 2002 In this context, the coral biostromes and coral mounds, bearing Operculina, Discocyclina and Asterocyclina, clearly represent in-situ production and accumulation in meso-oligophotic conditions, below the zone inhabited by Nummulites (Mateu-Vicens et al, 2012). The abundance of mud in the matrix, along with the absence of sediment sorting, reflects accumulation below storm-wave base.…”

“…Since their first occurrence, seagrasses have played a major role in carbonate production and sedimentation. These flowering plants sustain a prolific carbonate factory, as many of their epibionts possess a calcareous skeleton (e.g., coralline algae, bryozoans, mollusks, serpulid polychaetes, and foraminifers; Brasier, 1975;Eva, 1980;Ahr, 1997, 2006;Mateu-Vicens et al, 2012). Only exceptionally, seagrasses may directly precipitate aragonite (Enríquez and Schubert, 2014).…”

The Ainsa Fold and thrust oblique zone of the central Pyrenees: Kinematics of a curved contractional system from paleomagnetic and structural data

“…The youngest marine sediments in the studied area correspond with the Buil Nummulitic Banks at the bottom of the Escanilla Fm. [ Mateu‐Vicens et al ., ]. Above, fluvial sediments of the Escanilla Fm.…”