Abstract
The Morro do Chaves Formation (Barremian-Aptian of the Sergipe-Alagoas Basin) presents a sedimentary succession of approximately 150 m of thickness, composed of coquinas. Despite the sedimentological, stratigraphic, and petrophysical studies developed in coquinas, the general understanding of the relationship between sedimentary processes and meteoric diagenesis is not clear. The aim of this study is to recognize and understand the features of meteoric diagenesis in bioclastic from a well core drilled at Atol quarry. The core is composed of calcarenites and calcirudites interbedded with sandstones and shales. Taphonomic signatures show a low degree of physical alteration, without shells in life position. The main features related to meteoric diagenesis are bioclasts with bioerosive macrostructures (Caulostrepsis isp. and Oichnus isp.) and destructive micritization process. This was the main alteration process identified in the bioclasts and occurs throughout the coquinas interval and, in some beds, it was possible to distinguish three different stages of micritization. The differentiation of the stages results from different periods of variation of water depth, intensity of microorganisms attack, time of alteration, and variations in siliciclastic input. Additionally, one interval exhibits extremely micritized shells, curved fissures, and carbonate nodules, interpreted as having been deposited and subsequently altered in a palustrine environment.
Highlights
Coquinas are very valuable sources of paleontological, paleoecological, and sedimentological data (Kidwell et al 1986, Kidwell and Holland 1991, Simões et al 2000)
Physical and chemical factors influence preservation, deposits formed of coquinas are relatively common in the geological record from several examples — e.g., Vectis Formation, Lower Cretaceous (Radley and Barker 2000); Lagoa Feia Group, Lower Cretaceous (Muniz 2013); bioclastic accumulation of coastal plain of Rio de Janeiro State, Holocene (Castro 2009, Porto-Barros et al 2017), and Hamelin Pool, Holocene of Shark Bay (Playford et al 2013)
The results showed the bioclasts are predominantly non-articulated, more than 75% of the described beds have a rare occurrence of articulated bioclasts (Fig. 6A); more than 85% of bioclasts have some degree of fragmentation (Fig. 6B); rounded shells are not common with 55% of the bioclasts, showing absence of rounding and 38% exhibit a low degree of rounding (Fig. 6C); 60% of bioclasts have abrasion with a high degree of damage (Fig. 6D); 56% of the bioclasts have a low degree of preserved microarchitecture (Fig. 6E); 70% of the beds that compose the interval have shells with concave-up orientation and 5% are of convexity-up (Fig. 6F)
Summary
Coquinas are very valuable sources of paleontological, paleoecological, and sedimentological data (Kidwell et al 1986, Kidwell and Holland 1991, Simões et al 2000). Coquinas are defined, according to Kidwell et al (1986), as “any dense accumulation of hard parts of biogenic origin, independent of taxonomic composition, preservation state or degree of post-mortem modification”. They can be composed of fragmented or whole shells, or a mixture of them (e.g., Simões and Kowalewski 1998, Best and Kidwell 2000, Fürsich and Pandey 2003, Neves et al 2010, Ponciano et al 2012). Physical and chemical factors influence preservation, deposits formed of coquinas are relatively common in the geological record from several examples — e.g., Vectis Formation, Lower Cretaceous (Radley and Barker 2000); Lagoa Feia Group, Lower Cretaceous (Muniz 2013); bioclastic accumulation of coastal plain of Rio de Janeiro State, Holocene (Castro 2009, Porto-Barros et al 2017), and Hamelin Pool, Holocene of Shark Bay (Playford et al 2013)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
