Abstract:
A continuous section of Pliocene marine sediments was recovered
at Ocean Drilling Program Site 978, located in the Alboran
Sea between Spain and Morocco. Three Pliocene subunits have been
defined at Site 978: the lowermost (Subunit IC, 129.2 m thick) is characterized
by alternating beds of lighter, more calcareous, and darker,
less calcareous, claystone with bioturbated upper and lower contacts
(Type 1 cycles); the middle (Subunit IB, 67.1 m thick) is composed of
relatively homogeneous nannofossil claystone; and the uppermost
(Subunit IA, 211.6 m thick) contains abrupt-based darker, terrigenous
layers interpreted as turbidites that are interstratified with lighter nannofossil
claystone (Type 2 cycles). The rhythmically bedded light and
dark layers in Subunit IC correlate with those in the Rosello Composite
Section of Sicily, a global reference standard for the Pliocene time
scale. These sedimentary cycles are products of variations in precession
and resulting continental runoff. Missing cycles occur during eustatic
highstands. The shift to more homogeneous sedimentation in Subunit
IB is represented in similar-aged sequences throughout the Mediterranean
which display evidence of submarine mass wasting. Mediterranean-
wide slope degradation was likely a response to rapid sea-level
change at approximately 3 Ma. This change in sedimentation style was
accompanied by an upsection increase in sediment accumulation rates
associated with turbidite influx in Subunit IA. Turbidite frequency
throughout the Pliocene section can be linked to eustatic changes in
sea level, with turbidite maxima corresponding with mid-sequence
downlap surfaces and their associated condensed sections.
