UTRECHT MICROPALEONTOLOGICAL BUllETINS

sedimentation have not been found. The hardground-like character at the
top of the indurated limestone part of the cycles was not found to be
correlated with any notable change in benthonic fauna that might indicate
sudden changes in bottom conditions.
Upwards, the supply of clay seems to decrease, allowing burrowers to
homogenize the sediment again in 1 to 1.2 m thick beds. More marly intercalations
of only 20 to 40 cm indicate that the supply of fines had not
ceased completely. Before the onset of a different type of sedimentation
the supply of terrigenous fines briefly increased over an interval of only
1.6 m.
The various brown coloured, pyrite containing subhorizontal levels
within the thick limestone bed at the base of the laminated/non-laminated
part of the section may have been formed as a result of negative Eh conditions
below the sediment-water interface, caused for instance by high quantities
of decomposing organic detritus (cf. Brolsma, 1975b, p. 362). Similar
material fills the U-shaped burrows in which comparable reactions have
probably played a role. Finely bedded sediments appear to be associated
with similar pyritic inclusions in the overlying bed. Burrowing organisms
evidently did not destroy this fine bedding. High organic productivity in
the overlying water column may have enhanced the subsurface formation
of pyrite and obstructed the activity of burrowers. Oxygen depletion below
the sediment-water interface may at times have extended up to this interface,
resulting in still further deteriorating bottom conditions.
The sack-like structures in the overlying bed are interpreted as the result
of loadcasting. The slight lateral component probably was caused by gliding
movements toward the south east, as is concluded from the orientation of
the axial planes of the loadcasts. These features indicate rapid deposition as
well as a dip of the seafloor toward the south east. This possibly indicates
that the deeper part of the basin was located SE of Capo Rossello.
The gradual transition at the base of the first laminated interval of substantial
thickness may be best explained by an alternation of periods of
higher productivity increasing the rate of sedimentation, and periods of
lower productivity and consequently lower sedimentation rates. The intensity
of bottom life varied accordingly but never ceased completely. Even the
finest laminated parts show small burrows which do not disturb the lamination.
Such laminated or finely bedded sediments may even be present in the
more indurated, seemingly homogeneous limestones as thin intercalations,
which indicates the fluctuating character of the poor bottom conditions in
the course of time.
Occasional oblique or subhorizontal burrows of greater dimensions indi-