Climatic and tectonic controls on carbonate deposition in syn‐rift siliciclastic fluvial systems: A case of microbialites and associated facies in the Late Jurassic

Abstract

This work provides new insights to assess the factors controlling carbonate deposition in the siliciclastic fluvial systems of rift basins. Sedimentological and stable-isotope data of microbialites and associated carbonate facies, along with regional geological information, are shown to reveal the influence of climate and tectonics on the occurrence and attributes of carbonate deposits in these settings. The Vega Formation -a 150 m thick Lower Kimmeridgian siliciclastic fluvial sequence in Asturias Province (northern Spain) -constitutes a candidate for this approach. This unit includes varied facies (stromatolites; rudstones, packstones and wackestones containing oncoids, intraclasts, charophytes and shell bioclasts; marlstones and polygenic calcareous conglomerates) that formed in a low-gradient fluvial-lacustrine system consisting of shallow, low-sinuosity oncoid-bearing channels and pools within marshy areas, with sporadic coarse alluvial deposition. The sedimentological attributes indicate common erosion by channel overflow and rapid lateral changes of subenvironments caused by water-discharge variations. The carbonate fluvial-lacustrine system developed near uplifted marine Jurassic rocks. The occurrence of the system was conditioned by normal faults (active during the deposition of the unit) that favoured: (i) springs of HCO 3 -Ca-rich water from a Rhaetian-Sinemurian carbonate rock aquifer; and (ii) carbonate deposition in areas partially isolated from the adjacent siliciclastic fluvial system. The microbialite d 13 C and d 18 O values support deposition in a hydrologically open system, fed by ambient-temperature meteoric water, with riparian vegetation. Three types of lamination in the stromatolites and oncoids reflect distinct morphological types of cyanobacterial communities. The textural pattern of lamination parallels d 13 C and d18 O changes, suggesting short-term cycles of precipitation and temperature. A moderately to strongly contrasted seasonal and/or pluriannual precipitation regime is inferred from the cyclic d 13 C pattern of the lamination and from the discontinuous and asymmetrical growth of oncoids. Thus, the isotopic and sedimentological attributes of the carbonate deposits were linked to short-term climate changes associated with semi-arid conditions, consistent with the studied climatic zone.

Temporal and environmental significance of microbial lamination: Insights from Recent fluvial stromatolites in the River Piedra, Spain

“…In contrast, Arenas et al . (), based on textural and stable‐isotope values, tentatively proposed a seasonal to pluriannual duration of the dark and light composite laminae found in oncolites that developed in a Jurassic fluvial rift basin in northern Spain. Using stable isotopes, Brasier et al .…”

“…Following Arenas et al . (, ), the term ‘composite lamina’ refers to a group of two or more laminae that is distinguished from the underlying and/or overlying deposits by changes in lamina thickness, colour and/or texture (Fig. ).…”

“…Following Arenas et al (2007Arenas et al ( , 2015, the term 'composite lamina' refers to a group of two or more laminae that is distinguished from the Table 1. Mean flow velocity, water depth and deposition rates obtained from November 1999 to September 2012 at the study sites with fast-flow and stromatolite formation in the River Piedra (compiled from Arenas et al, 2014).…”

“…Most stromatolites and oncolites encompass more than one type of lamination, based on the repetition pattern and/or the textural components (Monty, 1976;Casanova, 1994;Arenas et al, 2007;Su arez-Gonz alez et al, 2014) and the rank of cyclicity (Lindqvist, 1994;Seong-Joo et al, 2000;Storrie-Lombardi & Awramik, 2006;Petryshyn et al, 2012;Arenas et al, 2015). With respect to stromatolites, most studies have focussed on the environmental and temporal significance of the laminae based largely on their thickness and textural features (Casanova, 1994;Seong-Joo et al, 2000;Storrie-Lombardi & Awramik, 2006;Su arez-Gonz alez et al, 2014).…”

“…Stable isotope (d 13 C and d 18 O) records from laminated microbialites commonly reveal climatic and associated hydrological changes on various time scales (i.e. seasonal and pluriseasonal; Andrews & Brasier, 2005;Kano et al, 2007;Kremer et al, 2008;Os acar et al, 2013;Tang et al, 2014;Arenas et al, 2015). Although the recognition of such changes based on textural attributes and/or thicknesses of the laminae is not always straightforward (e.g.…”