Marine or freshwater? Accessing the paleoenvironmental parameters of the Caldas Bed, a key marker bed in the Crato Formation (Araripe Basin, NE Brazil)

Abstract

The Aptian Crato Formation is world renowned for its well-preserved fossils in microbially-induced laminated limestones, which are regarded as one of the main Cretaceous Konservat-Lagerstätte of the geological record. Detailed stratigraphic investigation and mapping of the up to 90-m-thick Crato Formation at the eastern border of the Araripe Plateau allowed recognition of a regionally persistent fossil-bearing muddy interval, herein defined as the Caldas Bed. At its type locality, it is defined as an up to 2-m-thick coarsening-upward succession of grey/green mudstone and interbedded sandy siltstone and claystone. The 0.85-to 2-m-thick interval was recognized in several localities along the outcrop belt, and it is bounded by sharp, lower (Konservat-Lagerstätte limestone) and upper (sandstone and heterolithic facies) contacts. Despite previous literature data suggesting the presence of marine mollusks, the bed contains freshwater bivalves, small gastropods, spinicaudatans, plant remains, trace fossils, and rare ostracods. The Caldas Bed records benthic paleocommunities representing a short-term isochronous regional freshening event, marked by abrupt changes in sedimentation pattern, bathymetry, salinity, oxygenation and water chemistry.

Organic and Isotopic Geochemistry of Evaporites and Shales of the Santana Group (Araripe Basin, Brazil): Clues on the Evolution of Depositional Systems and Global Correlation during the Lower Cretaceous

“…The Barbalha and Crato formations showed TOC/TS ratios suggesting deposition in a mainly freshwater lacustrine depositional environment, a result in agreement with most of the former research (i.e., [41,[102][103][104]). Usually, the sulfur isotopic data are also consistent with oxic conditions, a finding that also match recent work by Varejão et al (2021) [105] who interpreted the Barbalha-Crato interval as deposited in a dominantly freshwater lacustrine system due to the abundance of non-marine fossils and an oxygen isotopic ratio (δ 18 OV-PDB) between −6.9‰ and −4.48‰.…”

“…Recent works [27,107] also point that the evaporites of the Ipubi Formation may have putatively precipitated by the evaporation of marine waters. These findings, as in the present work, point for intermittent marine incursions in the Ipubi Formation [105] rather than an isolated playa-lake environment [58], in a probable true coastal sabkha system [48].…”

“…The salinity attests for waters under brackish conditions with marine influence (Figure 5). Marine behavior for the Romualdo Formation is well documented (i.e., [48,88]), but only recently have some works gathered evidence for a true epicontinental sea [105,108].…”

Mixed siliciclastic–carbonate sedimentation in an evolving epicontinental sea: Aptian record of marginal marine settings in the interior basins of north‐eastern Brazil

“…This facies association is characterized by an up to 2 m thick thickening‐upward succession of mudstone, and interbedded sandy siltstone and claystone laterally continuous for tens of kilometres (Fig. 9A), also known as the Caldas Bed (Varejão et al ., 2021). Up to 1 m thick tabular beds of massive to poorly laminated, green to yellow coloured mudstone occur at the base (Fig.…”

“…The muddy succession is interpreted to be deposited in a freshwater lacustrine system based on the abundance of non‐marine forms (for example, Unionida; Silva et al ., 2020a,b) and the oxygen isotopic composition (Varejão et al ., 2021). Low δ 13 C values suggest lower primary productivity in surface waters, compared to those observed in FA‐4 (see Armstrong & Brasier, 2005).…”