Taphonomic Processes in Selected Lower and Middle Jurassic Foraminifera From the Iberian Range and Basque-Cantabrian Basin (Spain)

Abstract

Lower and Middle Jurassic foraminifera from the Iberian and Basque-Cantabrian Basins exhibit distinct features due to different biostratinomic and diagenetic processes. The main mechanisms of taphonomic alteration identified in our material are bioerosion, abrasion, fragmentation, dissolution, sedimentary infilling, cementation, mineralization, recrystallization sensu lato, pressure-dissolution processes, distortion and removal. Sample preparation prevents the detection of mechanism such as reorientation, regrouping or geopetal infillings. Bioerosion is represented by rare and isolated holes. Pitted surfaces associated with wall pores are the most important corrasion features. Abrasion signs such as polished longitudinal and sutural ribs have been recognized. Fragmentation is common and it is controlled by the shape of the test rather than by the shell size or thickness. Different types and patterns of dissolution have been detected. Micritic infillings have been observed in most of the specimens; nevertheless, sparitic and pyrite cementations are also abundant. Neomorphism includes features such as inversions and recrystallization; pyrite and possibly silica replacements have been recognized. A great number of the specimens show calcitic crusts that obscure wall surfaces and may prevent identification. Mineralization is represented by sylvite crystals on the surface of some uppermost Toarcian and Aalenian foraminiferal shells. Fossil diagenetic distortion is frequent and affects the specimens independently of their morphology. Signals of reworking (''resedimentation'') are common in the specimens. Two direct criteria for the recognition of reworking (''reelaboration'') are shown for the first time on the foraminiferal shells. More research must be done to discern the history of the foraminiferal assemblages in order to achieve accurate biostratigraphical, paleoecological and paleobiogeographical interpretations.

Foraminifera of the Toarcian-Aalenian boundary from the Lusitanian Basin (Portugal): a paleoecological analysis

“…No evidence of taphonomic processes that could modify the composition of the original assemblage has been recognized. In a few cases, determinations were limited to the generic level, or even to the family level, due to fragmentation, partial or total dissolution of the shells (mainly affecting the aragonitic representatives of the family Ceratobuliminidae) or significant recrystallization (Herrero and Canales, 2002), the last one affecting more intensively the Zambujal de Alcaria specimens. As a consequence, the specimens of foraminifera from this section are poorly preserved when compared with the specimens obtained in the Maria Pares section, as can be seen in figures 4, 5 and 6, where the most representative taxa are illustrated.…”

Depositional environment and biofacies characterization of the Upper Pennsylvanian–Lower Permian deposits of the San Salvador Patlanoaya section (Puebla, Mexico)

“…The taphonomic characteristics of the faunal assemblages collected in the Paltlanoaya section were detailed according to the previous studies of Fernµndez-López (2000), Herrero and Canales (2002) and Cózar (2003). The preservational features are relatively uniform in the San Salvador Patlanoaya section.…”

“…Bioerosion, encrustation, corrosion, and dissolution of tests (for references and definitions, see Herrero and Canales 2002) appear to be scarce. This weak alteration of the foraminifera is interpreted as due to rather rapid burial and a habitat near the site of final burial (see also Malinky and Heckel 1998).…”

“…the staffellid Nankinella, is present only at one level in the Upper Missourian. Robust Triticites are commonly transported as bed load, smaller foraminifera and remains of the alga Epimastoporella by suspension load (Herrero and Canales 2002). The vicissitudes of Recent Alveolinella are probably a model for Triticites (Severin and Lipps 1989;Murray 1991;Holcovµ 1999;Yordanova and Hohenegger 2002).…”

Internal pore measurements on macroperforate planktonic Foraminifera as an alternative morphometric approach

“…Bé (1968) noted that pores should be measured on the internal side of the shell, as the outer side is often obscured. In particular in the fossil record, Foraminifera are frequently poorly preserved (e.g., Spezzaferri 1995;Herrero and Canales 2002), and this seriously hampers the identification of species and even more a precise analysis of the outer shell ultrastructure. Indeed, in many cases, calcification (e.g., Hemleben et al 1989;Caron et al 1990), recrystallization (e.g., Sexton and Wilson 2009), test dissolution (Hemleben and Olsson 2006;Schiebel et al 2007;Johnstone et al 2010;Nguyen et al 2011), diagenetic alteration (Hemleben and Olsson 2006) considerably affect the ultrastructure including pore diameters, and even the general test morphology.…”