TAPHONOMIC EFFECTS ON THE PALEOECOLOGICAL RECORD OF THE LOWER PLEISTOCENE SHALLOW BENTHIC FORAMINIFERA ASSEMBLAGES (WESTERN EMILIA, ITALY)

Abstract

Stefanelli S.,Taphonomic effects on the paleoecological record of the lower Pleistocene shallow benthic foraminifera
assemblages (Western Italy). (IT ISSN 0394 – 3356, 2004).
Taphonomic and paleoecological quantitative analyses are drawn from benthic foraminiferal assemblages from four lower Pleistocene
marine sections (Western Emilia). Then, the data are used to perform the Cluster Analysis (by means SPPS Program) in order to group
samples and benthic foraminifera on the basis of their taphonomic and paleoecological similarities.
The sections are arranged in meter to tens-of-meters thick sedimentary cycles. The lower part of each cycle represents the sedimentological transport of shelly material from shallow sandy bottoms to slightly deeper settings (20-40m depth range) due to catastrophic flooding events. The upper part of the cycles represents the return to normal, muddy deposition in an inner shelf setting.
For the taphonomic analysis, foraminifera specimens are classified into taphonomic categories on the basis of their state of preservation: (1) fresh tests, (2) opaque tests, (3) light orange to dark brown iron-stained tests, (4) black or black-striped tests filled with or replaced by pyrite (including partial dissolution of the tests), (5) breakage of the tests.
In both the lithological intervals of the cycles, the high number of specimens grouped into “breakage tests” category leads to suppose
the destruction of the tests as the principal taphonomic process affecting the foraminiferal assemblages. In the mudstone partings of the sandstone interval, the mechanism that caused the destruction of the tests is the transport in a shallow-high hydrodynamic setting
where benthic foraminifera are put into suspension in water and collide each other and/or with sediment grains. In the same interval,
paleoecological results points out that the specimens grouped into “fresh tests” category belong to displaced taxa coming from shallow sediments due to the transport. As a consequence, a correct paleoecological interpretation from the assemblages of the lower part of the cycles is not possible.
In the high-bioturbated mudstones, the “breakage tests” category consists in fragments of hyaline specimens. In these lithological intervals of the cycles, the destruction of the tests may well be due to a strong dissolution activity that causes, in the typical dissolution
sequence of pore-boaring species, the corrosion and amplification of the pores and, then, the destruction of the chambers. Though the taphonomic activity, a paleoecological interpretation is possible. It reconstructs a sedimentary deposition in a shallow-quite environment and changes in oxygen bottom-water content.