Abstract
The geometry of carbonate platforms reflects the interaction of several factors. However, the impact of carbonate-producing organisms has been poorly investigated so far. This study applies stratigraphic forward modelling (SFM) and sensitivity analysis to examine, referenced to the Miocene Llucmajor Platform, the effect of changes of dominant biotic production in the oligophotic and euphotic zones on platform geometry. Our results show that the complex interplay of carbonate production rates, bathymetry and variations in accommodation space control the platform geometry. The main driver of progradation is the oligophotic production of rhodalgal sediments during the lowstands. This study demonstrates that platform geometry and internal architecture varies significantly according to the interaction of the predominant carbonate-producing biotas. The input parameters for this study are based on well-understood Miocene carbonate biotas with characteristic euphotic, oligophotic and photo-independent carbonate production in which it is crucial that each carbonate-producing class is modelled explicitly within the simulation run and not averaged with a single carbonate production–depth profile. This is important in subsurface exploration studies based on stratigraphic forward models where the overall platform geometry may be approximated through calibration runs, and constrained by seismic surveys and wellbores. However, the internal architecture is likely to be oversimplified without an in-depth understanding of the target carbonate system and a transfer to forward modelling parameters.
The evolution of carbonate systems has traditionally been linked to relative sea level and regional tectonics as the first-order controls over geometries and facies associations (Kendall and Schlager 1981; Handford and Loucks 1993). This is in concert with climate and ocean circulation, which affect temperature, nutrients, ocean chemistry and the type of carbonate-producing organisms through time (Milliman et al. 1974; Stanley and Hardie 1998; Mutti 2019). Many studies have focused on how these boundary conditions impact carbonate-producing organisms (e.g. Chave 1967; Lees and Buller 1972; Carannante et al. 1988; Nelson 1988; James and Clarke 1997; Mutti and Hallock 2003), and how these translate into geometrical and facies properties of carbonate systems and carbonate factories (Schlager 1992, 2005; James and Clarke 1997). Geometries of carbonate systems, ranging from reef-rimmed to homoclinal ramps and a range of forms in between (Ahr 1973; Ginsburg and James 1974; Read 1985; Handford and Loucks 1993; Williams et al. 2011), can be linked to dominant biota at any given time during the evolution of the platform (e.g. James and Bone 1991; James and Clarke 1997; Pomar et al. 2012a). Based on ecological and depositional requirements, carbonate-sediment-producing biotas have been grouped into various categories (Purdy 1963; Lees and Buller 1972; Lees 1975; Carannante et al. 1988; James and Clarke 1997; Pomar 2001a, 79b; Halfar et al. 2004; Schlager 2005; Wilson and Vecsei 2005; Roberts and Bally 2012; Michel et al. 2018; Brandano et al. 2019). Due to the ability of many euphotic carbonate-producing biota, such as hermatypic corals, to withstand wave action, they are able to construct rimmed platforms, while the creation of low-angle ramps by many known oligophotic biota is a consequence of their susceptibility to dispersion by currents (Pomar 2001b; Williams et al. 2011). Several authors have described the transition from ramp to rimmed platforms and vice versa (e.g. Biddle et al. 1992; Pomar 2001a; Schlager 2005; Benisek et al. 2009; Tomás et al. 2010), and have highlighted that changes to the availability of accommodation space is the driving force in initiating this ramp to rimmed-reef transition. However, they have also recognized that a shift in platform geometry might be linked to the temporal evolution of the dominant carbonate factory (Pomar 2001a; Pomar and Hallock 2007; Janson et al. 2010; Pomar et al. 2012a). This case study highlights how architectural changes occur in synchrony with a shift of the predominant carbonate factory at a given time and can be related to changes in the environmental conditions that cause a shift from oligophotic production to euphotic sediment production or vice versa.
Pomar and Haq (2016) showcased how the interaction of different factors specific to carbonate systems results in a variety of platforms by contrasting sequence-stratigraphic responses in clastic and carbonate environments. Different biota and their respective carbonate factories have the capacity to control platform geometry and facies dynamics through ecological constraints on the platform growth rate (James and Bone 1991; Burchette and Wright 1992; Williams et al. 2011). Furthermore, the complex interplay between coexisting and competing carbonate biota in a given system has a first-order control on sediment rigidity (e.g. framework builders v. sediment grains), grain size, and the shape of produced sediments and their spatial and temporal arrangements (Pomar 2001a; Williams et al. 2011). Analysis of these processes and their role in shaping carbonate geometries is difficult to assess in a quantitative way due to the sparse number of datasets from field observations. Recent advances in stratigraphic forward modelling (SFM) of carbonate systems (Granjeon and Joseph 1999; Warrlich et al. 2002; Burgess et al. 2006, 2013; Williams et al. 2011; Hawie et al. 2015; Kolodka et al. 2016; Salles et al. 2018a; Zhang et al. 2019; Li et al. 2020; Pall et al. 2020) offer a powerful tool for quantitative analysis of carbonate-system development through time and space, and for testing the impact of different controls over platform evolution. Extensive research using SFM has been undertaken in recent years (Burgess et al. 2006; Williams et al. 2011; Nader et al. 2018; Salles et al. 2018b; Al-Salmi et al. 2019; Hawie et al. 2019; Sultana et al. 2022), yet there has not been a significant focus on numerical modelling investigating the role of carbonate-producing biota on platform geometry.
The aim of this study is to analyse quantitatively the effect of temporal and spatial change of the dominant carbonate-producing organisms and their differential rates of production on the resultant platform geometry, and to investigate how different carbonate factories interact to generate depositional geometries by employing SFM. We focused our modelling on: (1) investigating the sensitivity of platform geometries to biotic sediment production; (2) understanding how euphotic v. oligophotic carbonate production drives changes in platform geometry; and (3) how changes in initial bathymetry (at the onset of platform development) influence platform geometry, especially the extent of platform progradation.
We chose the Miocene Llucmajor carbonate system of Mallorca (Spain, Western Mediterranean) as a reference for this study, as the setting has been the focus of numerous and fundamental studies addressing the controls over stratigraphic and facies architecture and its evolution (Esteban 1979; Pomar 1993; Pomar and Ward 1994, 1995; Pomar et al. 1996, 2012b; Pomar and Hallock 2007; Sàbat et al. 2011). The platform is a reef-rimmed carbonate system built by reef-building euphotic corals and green algae in the shallow-water and upper slope domain, and oligophotic rhodalgal communities in the deeper slope (Carannante et al. 1988; Pomar et al. 2012a), both interacting as sea level fluctuates to create the platform progradational and aggradational sequences. The superbly exposed and extensively studied carbonate sequences allow direct validation and calibration of the modelling results. Two numerical models for the platform architecture and its facies distribution of the platform have hitherto been created (Bosence et al. 1994; Hüssner et al. 2001). The model of Bosence et al. (1994) provides a numerical representation of the platform architecture and, consequently, a stratigraphy-based interpretation of the episodic development of the facies as sea level fluctuates. Hüssner et al. (2001) used superimposed multiple synthetic sea-level curves to simulate the platform development and internal geometry with a focus on achieving a close match with the outcrop data. However, both models did not address the impact of different carbonate biota on the evolution of the platform. The study presented here focuses on the role and impact of biotic carbonate production by creating a stratigraphic forward model constrained by published outcrop data (Pomar 1991, 1993, 2020; Pomar and Ward 1995; Pomar et al. 1996; Pomar and Haq 2016) with a series of sensitivity analyses. The latter was used to investigate the possible platform responses that would result under varied conditions of carbonate production and accommodation space. We herein present modelling-based evidence for the impact of differential carbonate-production rates (with a focus on euphotic corals v. oligophotic rhodalgal sediment production) on platform geometries, and how these interact spatially and temporally with changing accommodation space to define carbonate platform architecture.
