Aptian carbonate platform development in the Southern Iberian Palaeomargin (Prebetic of Alicante, SE Spain)

Abstract

The Aptian stratigraphic record of the Alicante region consists of: a rudist and coral-rich carbonate platform of earliest Aptian age (Llopis Formation), with a discontinuous siliciclastic member at its top; followed by late Early, to Late Aptian hemipelagic marls and marlstones (Almadich Formation); and then by renewed carbonate platform development of Late Aptian to earliest Albian age (Seguilí Formation). In the Llopis Formation, SW-dipping, massive clinoform beds of bioclastic debris are succeeded by flat-lying platform-top beds. The latter show a cyclically regressive stacking of biofacies, with rudist-dominated floatstone in their lower parts passing upwards to finer-grained, more sparsely fossiliferous bed tops with burrow mottling. Caprinid rudists, with originally almost wholly aragonitic shells, dominate the external platform-top facies, while more internal facies contain a mix of monopleurid, polyconitid and requieniid rudists, all with relatively slightly thicker development of the calcitic outer shell layer, together with caprinids. Biostratigraphic and carbon-isotope data link the termination of the Llopis platform with the onset of OAE1a. The carbonate platform of the Seguilí Formation again contains tabular platform-top beds showing repeated cyclic regression, with dense rudist and/or chondrodont floatstones overlain by sparser floatstones with wackestone matrix and secondarily filled burrows. But caprinids are now absent, while requieniids and polyconitids, some of large size, as well as radiolitids, all with thickened calcitic outer shell layers, accompany the tubular monopleurid, Mathesia, together with a greater development of Chondrodonta biofacies. The same overall pattern of biotic turnover from the Early, to the Late Aptian is confirmed in other parts of Iberia and contiguous regions. Moreover, Iberian platforms of late Early Aptian age outside the present study area reveal a transitional phase with an increasing proportion of polyconitids in the outer platform-top to upper slope facies at the expense of caprinids. The siliciclastic influx at the top of the Llopis Formation implies a climatic shift from arid, to relatively more humid/pluvial conditions through the mid-Early Aptian, as seen in several other Iberian sections. This climatic change was probably forced by the intensified greenhouse conditions at the onset of OAE1a. By contrast with these Iberian platforms, caprinids continued to dominate the outer platform-top zones of some central to southern Tethyan platforms until the close of the Early Aptian. This broad palaeolatitudinal differentiation of rudist associations within the Tethyan belt implies a climatic influence, whether exerted through thermal modulation of seawater pH and/or aragonite saturation, variation in nutrient flux, or any combination of these. Keywords: Aptian / carbonate platforms / Prebetic Zone / palaeoecology / rudist coral formations / OAE1aRésumé -Développement des plates-formes carbonatées dans l'Aptien de la marge continentale sud-Ibérique (Prébétique d'A...

High‐Resolution C‐Isotope, TOC and Biostratigraphic Records of OAE 1a (Aptian) From an Expanded Hemipelagic Cored Succession, Western Tethys: A New Stratigraphic Reference for Global Correlation and Paleoenvironmental Reconstruction

“…Carbonate C‐isotope records serve as an excellent tracer for the history of the global carbon cycle (e.g., Arthur et al., 1985; Cramer & Jarvis, 2020; Jenkyns, 2010; Weissert, 1989, 2019). The negative and positive spikes and shifts in the Aptian C‐isotope record are linked to changes in p CO 2 (e.g., Jarvis et al., 2015, 2011; Méhay et al., 2009; Menegatti et al., 1998; Naafs et al., 2016), temperature (e.g., Dumitrescu et al., 2006; Jarvis et al., 2015; Jenkyns, 2018; Kuhnt et al., 2011; Naafs & Pancost, 2016), ocean fertility (e.g., Aguado et al., 2014a, 2016, 2017, 2008; Bottini & Erba, 2018; Bottini et al., 2015; Herrle et al., 2010; Mutterlose & Bottini, 2013), carbonate platform initiation and drowning (e.g., Huck et al., 2011, 2013, 2010; Masse & Fenerci‐Masse, 2013; Skelton & Gili, 2012; Skelton et al., 2019) and biotic crises (e.g., nannoconid crisis, Erba, 1994; planktonic foraminiferal and radiolarian turnover, Erbacher & Thurow, 1997; Leckie et al., 2002) or changes in the rudist fauna (Skelton & Gili, 2012). However, despite the dramatic short‐term and longer‐term shifts recorded in lower Aptian records, no prominent extinction events occurred during the early Aptian, probably due to the increase in the resilience of the Cretaceous biosphere (Weissert, 2019).…”

“…Interestingly, the neighbouring shallow‐water carbonate platform area recorded a demise coeval with the onset of OAE 1a, followed by deposition of siliciclastics, which is interpreted as the result of an abrupt environmental perturbation (Castro et al., 2008; Skelton et al., 2019). Biostratigraphic and C‐isotope data (Skelton et al., 2019) are consistent with a correlation between the major negative shift in δ 13 C recorded in the Cau core and the demise of the platform. These observations agree with data published from the Basque‐Cantabrian basin (Millan et al., 2009, 2011).…”

“…The studied section was located at a paleolatitude of 20–25°N (Masse et al., 1993; Figure 1c), interpreted to lie within the northern part of the equatorial arid belt (Chumakov et al., 1995). The early Aptian sedimentary evolution of the SICM was impacted by OAE 1a; shallow platforms were subject to an extensive drowning event (e.g., Castro et al., 2014, 2008; Skelton et al., 2019), whereas in outer platform and pelagic settings OAE 1a was marked by the deposition of organic‐rich sediments (e.g., Aguado et al., 2014a; de Gea de et al., 2003, 2008).…”

“…Some of the most remarkable environmental changes recorded in Aptian sedimentary successions occurred during Oceanic Anoxic Event 1a (OAE 1a; Arthur et al., 1990), which saw widespread elevated burial rates of organic matter in oxygen‐depleted ocean bottom waters and/or within expanded oxygen‐minimum zones (e.g., the Selli Level of Coccioni et al. (1987)), a nannoconid crisis (Erba, 1994; Erba et al., 2004), the growth and regional demise of carbonate platforms (Castro et al., 2008; Föllmi, 2012; Föllmi et al., 1994; Graziano, 2013; Huck et al., 2010, 2012; Immenhauser et al., 2005; Masse & Fenerci‐Masse, 2011; Ruiz‐Ortiz et al., 1998; Skelton & Gili., 2012; Skelton et al., 2019; Vahrenkamp, 2010; Weissert et al., 1998; Wissler et al., 2003), and rapid warming followed by cooling events, with possible ice age interludes during the late Aptian (e.g., Bottini & Erba, 2018; Bottini et al., 2015; Davies et al., 2020; Dumitrescu et al., 2006; Erba, 1994; Hochuli et al., 1999; Jenkyns, 2010, 2018; Maurer et al., 2013; McAnena et al., 2013; Naafs & Pancost, 2016; Weissert & Lini, 1991; Weissert et al., 1998). A rapid rise of atmospheric CO 2 has been invoked as a trigger for OAE 1a, which has been linked to submarine volcanic activity and possibly to methane emissions (Adloff et al., 2020; Bottini et al., 2012, 2015; Erba et al., 2010, 2015; Kuhnt et al., 2011; Leckie et al., 2002; Méhay et al., 2009; Naafs et al., 2016; Weissert & Erba, 2004).…”

“…This highlights the robustness of the C‐isotope stratigraphy of the Cau section as a reference for global correlation, and is the basis for discussion on the dynamics of the carbon cycle, environmental and biotic perturbations that occurred during the Aptian. A further advantage of the Cau section is the proximity of neighboring carbonate platform successions, the intimate stratigraphical relations of which with OAE 1a have also been investigated (Skelton et al., 2019).…”