The succession drilled and sampled on the shelf (Sites 1071 and 1072) is divisible into at least four unconformity-bounded sequences of late-middle Miocene to Pleistocene age. The presumed sequence boundaries are characterized in seismic reflection data by pronounced offlap and onlap; the sequences are arranged in a forestepping (overall progradational) pattern. Best estimates of ages for the unconformity surfaces are <0.5 Ma [pp3(s)], >1.1-<7.4 Ma [pp4(s)], 7.4-11.2 Ma [m0.5(s)], and >11.4 Ma [m1(s)]. The abbreviations "pp" and "m" refer to Pliocene-Pleistocene and Miocene, respectively, and the suffix "s" indicates that the surface was identified on the basis of seismic reflection geometry on the shelf. An additional surface, tentatively interpreted as a sequence boundary prior to the cruise [pp5(s)], is tenatively reinterpreted as a marine flooding surface and dated as 5.9-7.4 Ma (that is, Miocene rather than Pliocene-Pleistocene). The major unconformities are thought to represent times/intervals of falling sea level, with higher order cyclicity most likely present but not resolved with existing seismic reflection data. The succession consists largely of sands, silts, and clays with recovery predominantly from muddy intervals; the presence of sands in poorly recovered or unrecovered intervals is inferred from logging data. An unusual distribution of sediment types was observed between unconformity surfaces. The shallow shelf for each sedimentary unit between these surfaces is dominated by sediments that accumulated during overall flooding. Seaward of clinoform breakpoints/rollovers, the deeper shelf is dominated by sediments that accumulated when the shelf was building seaward.
Drilling on the slope (Site 1073) recovered a thick Pleistocene section and condensed Pliocene-late Eocene section with excellent biostratigraphic resolution. Postcruise studies, including detailed biostratigraphy and the tying-in of lithologic variations to the regional sequence stratigraphic framework, should provide additional constraints on the ages of surfaces identified on the shelf and recovered at Site 1073.
High-frequency, sea-level variations have apparently left an imprint on the chemistry of interstitial
waters recovered during Leg 174A. On the shelf, observed salinity (Cl-)
minima are consistent with alternate exposure of the shelf during the Pliocene-Pleistocene and
then renewed flooding by seawater. On the slope, five well-defined alkalinity maxima and four
HPO42- maxima in interstitial-water profiles from the thick late
Pleistocene succession can be attributed to variations in the amount or type of buried organic
matter between glacial and interglacial intervals.
To 174A Introduction
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