Sedimentology and taphonomy of sirenian remains from the Middle Eocene of the Pamplona Basin (Navarre, western Pyrenees)

Abstract

Sirenian vertebrae and ribs have been recently discovered from two Middle Eocene localities of the Pamplona Basin, Navarre (western Pyrenees). These outcrops correspond to different lower Bartonian lithostratigraphic units: the lower part of the Pamplona Marl Formation (Uztarrotz site) and the upper part of the Ardanatz Sandstone (Ardanatz site). The former represents a deep and low-energy sea floor far away from a deltaic slope; the Ardanatz environment probably corresponds to a semi-closed deltaic bay periodically affected by catastrophic floods (i.e., fluvial hyperpycnal flows). The presence of epibiontic activity suggests that the bones were exposed for a while prior to the burial. The histological structures are well preserved except in the peripheral region, where tubular-like microstructures filled by pyrite and iron oxides probably correspond to microbial bioerosion. The major mineral component of the fossil bones is francolite (carbonate fluorapatite). In the Ardanatz samples there is evidence of secondary francolite due to the late replacement of original carbonate fluorapatite through internal fractures. The Ardanatz and Uztarrotz sirenian fossils do not show any evidence of reelaboration. They have similar sum of rare earth elements (REE) concentrations relative to the host rock, but comparatively lower than in other vertebrate fossil bones. This feature may be due to the dense compact structure of pachyosteosclerotic sirenian bones.

Fossil associations from the middle and upper Eocene strata of the Pamplona Basin and surrounding areas (Navarre, western Pyrenees)

“…Peripheral zone of alteration with tunnel network structures, probably made by microbial activity, are observed in thin sections of fossil sirenian ribs (Astibia et al, 2005). Microfossils are often filled with pyrite.…”

“…2). The oldest Bartonian units (Erize Marl Formation, Ezkaba Sandstone Formation and Pamplona Marl Formation) still show sedimentological and palaeontological evidence of accumulation in relatively deep waters, and they were attributed to slope , turbiditic channel-levee (Payros et al, 1997) and prodelta (Astibia et al, 2005) environments, respectively. The overlying Ardanatz Sandstone Formation (also referred to as Gazolaz, Tajonar, Tabar and/or Gongolaz sandstone in the regional literature) has been thoroughly studied by Astibia et al (1999Astibia et al ( , 2005Astibia et al ( , 2006Astibia et al ( , 2014.…”

“…The oldest Bartonian units (Erize Marl Formation, Ezkaba Sandstone Formation and Pamplona Marl Formation) still show sedimentological and palaeontological evidence of accumulation in relatively deep waters, and they were attributed to slope , turbiditic channel-levee (Payros et al, 1997) and prodelta (Astibia et al, 2005) environments, respectively. The overlying Ardanatz Sandstone Formation (also referred to as Gazolaz, Tajonar, Tabar and/or Gongolaz sandstone in the regional literature) has been thoroughly studied by Astibia et al (1999Astibia et al ( , 2005Astibia et al ( , 2006Astibia et al ( , 2014. Silty marls are dominant in the locality of Ardanatz (central part of the study area), but they contain abundant tabular sandstone beds, which display sharp bases with occasional groove casts (indicating ENE palaeocurrent direction) and gradational upper boundaries; they are rich in carbonaceous plant remains, vertical and horizontal burrows (mainly Ophi- omorphids, sensu Seilacher, 2007) and bioclasts (e.g.…”

“…Microfossils are often filled with pyrite. Body macrofossils are generally fragmentary, with evidence of dissolution and replacement by calcite and celestite in sponges, dissolution and aragonite-calcite neomorfism processes in corals and molluscs, and replacement by well crystallized francolite (carbonate-fluorapatite) in fossil vertebrates (Astibia et al, 2005; (Fig. 3e, f).…”

“…Therefore, it is likely that the sirenian remains correspond to an ademic organism (sensu Fernández López, 1990;2000). Thus, the sirenian bones could have been derived from a floating carcass swept into the open sea (Astibia et al, 2005).…”