Tafonomy of macroinvertebrates and Albian marine ingression as recorded by the Romualdo Formation (Cretaceous, Araripe Basin, Brazil)

Prado, Ludmila Alves Cadeira do; Fambrini, Gelson Luís; Barreto, Alcina Magnólia Franca

doi:10.1590/2317-4889201820180048

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Brazilian Journal of Geology

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ARTICLE • Braz. J. Geol. 48 (03) • Jul-Sep 2018 • https://doi.org/10.1590/2317-4889201820180048 copy

Tafonomy of macroinvertebrates and Albian marine ingression as recorded by the Romualdo Formation (Cretaceous, Araripe Basin, Brazil)

Authorship SCIMAGO INSTITUTIONS RANKINGS

ABSTRACT:

The Romualdo Formation, Araripe Basin, crops out in the municipality of Exu, Pernambuco State, exposing coquinas. Fauna survey, taphonomic analysis of the macrobenthos, and identification of sedimentary facies of the outcrop were carried out to contribute to paleoenvironmental understanding of the Romualdo Formation. The association of molluscs (Cerithium sp., Tylostoma ranchariensis, Aguileria dissita, Brachidontes araripensis and unidentified cassiopids) and echinoids of the species Bothryopneustes araripensis suggests sedimentation in the marine environment. The taphonomic similarities made possible the grouping of coquinas into two types. Coquinas type 1 exhibit characteristics of transport and reworking of bioclast generated by storms in proximal setting. Coquinas type 2 demonstrates echinoids in situ and articulated with spines connected to shells, suggesting a fast sedimentation by distal tempestite. Regarding sedimentary facies, the concentrations of invertebrates would have been formed in the transitional offshore zone of siliclastic platform. The results presented here evidence a broader distribution of echinoids and expand the geographic limits of the shallow marine environment in the Romualdo Formation.

KEYWORDS:
echinoids; mollusks; tempestites; obrution deposits; South America

Facies Code	Facies	Description	Interpretation
Cq	Coquinas	Yellow, fine calcareous sandstones with remains of shells, mostly fragmented, sometimes with incipient truncated wavy laminations.	Accumulation in high-energy events. Storm beds (tempestites) in a shallow environment.
Fl	Laminated Shale	Light gray to whitish gray shales, laminated, layered with lateral continuity and centimetric thickness, with horizontal lamination and expressive scattered fossil content. There is often the occurrence of centimetric to decimetric discoidal calcareous concretions in the fossils, rich in remains of plants and fish.	Decanting of clays in calm waters. The light coloring suggests oxidizing conditions at the depositional site. Fall-out deposits.
Fml	Laminated mudstones	Laminated mudstones laterally persistent in succession with up to 1 m thick. Features light gray to whitish color and light tints (cream, yellow) when altered. It is common the presence of carbonate nodules and discoidal calcareous concretions associated or not to the presence of fossil. Remains of fish (scales, teeth, and bones), coprolites, mollusks and rare plant fragments are common.	Deposition by decantation in calm waters, disoxic or anoxic conditions. Fall-out deposits.
Sm	Massive sandstones	Tabular centimetric layers, or lenticular, of very fine to fine calcareous sandstones, well sorted.	Bottom currents associated with storm waves.
Sh	Laminated Sandstones	Very fine sandstones, well sorted, light gray to whitish gray color, with horizontal lamination, arranged in centimetric to decimetric tabular layers.	Deposition by bottom currents in plane beds under conditions of upper flow regime.
Sp	Planar cross-stratification sandstones	Fine sandstones with small and low angle planar cross-stratification, arranged in decimetric tabular layers. Color is light gray to whitish.	Migrating forms of sub-aqueous beds of sinuous crest due to continued action of unidirectional currents.

Coquinas	Taphonomic signatures	Interpretation
Type 1	High levels of fragmentation and or disarticulation, low abrasion and bioerosion and corrosion absent. Good sorted and loose to dense packed fossil assemblage. Bioclast orientation on bedding plans is concordant and polymodal, with the concavities facing both upwards and downwards. In cross section, chaotic distribution is less frequent. Presence of nesting of bivalves.	Transport and reworking of bioclastic through storm waves in proximal setting of transitional offshore zone.
Type 2	High levels of fragmentation and or disarticulation, low abrasion and abrasion and bioerosion and corrosion absent. Poor sorted and loose to disperse packed fossil assemblage. Bioclast orientation on bedding plans is concordant and polymodal, with the concavities facing both upwards and downwards. In cross section, chaotic distribution is less frequent. Presence of articulated echinoids with spines attached to the shells.	Rapid sedimentation of echinoids in situ by storms in more distal setting of transitional offshore zone.

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