AGE MODEL

The age model for the 549-m-thick lower Miocene to Pleistocene sequence cored at Site 1193 is well dated above 35 mbsf, poorly dated in the carbonate platform interval (35-229 mbsf), and moderately well dated below 229 mbsf (Table T7; Fig. F42) (see "Biostratigraphy and Paleoenvironments"). In the uppermost 35 mbsf, the hemipelagic Pliocene-Pleistocene Megasequence D, 11 calcareous nannofossils and four planktonic foraminifer datums provide a rather detailed biostratigraphy. Below 229 mbsf, the lower part of Megasequence B and Megasequence A, four calcareous nannofossil datums and two planktonic foraminifer datums offer low-resolution, early Miocene biostratigraphic control. The carbonate platform, from 35 mbsf to 229 mbsf, was barren of age-diagnostic planktonic microfossils. However, larger benthic foraminiferal assemblages in the entire sequence below 35 mbsf indicate an age of 12-24 Ma, constraining the time of platform buildup to 12-16 Ma (Fig. F42) (see "Biostratigraphy and Paleoenvironments"). This age range does not necessarily exclude younger platform growth that could have been eroded.

A sequence of magnetostratigraphic reversals in the sediments above the platform (0-35 mbsf) is interpreted so that it matches the biostratigraphic results in this interval. Below that depth, recovery was insufficient to allow development of a magnetostratigraphy. Because of the difficulties with the magnetostratigraphic record, the biostratigraphic control points are used to construct the shipboard age model for Site 1193 (Fig. F42).

Interval sedimentation rates range from 0 (hiatus) to almost 160 m/m.y. in the lower Miocene periplatform deposits. The most significant increase (one order of magnitude) in sedimentation rate occurs in the upper lower Miocene in lithologic Subunit VIA and can be interpreted as the onset of shedding from the prograding carbonate platform, augmented by clay from the continent and modified by current activity (see "Lithostratigraphy and Sedimentology"). In the latest Miocene, several million years after the demise of the carbonate platform, hemipelagic sedimentation began at Site 1193 and continued through the Pleistocene at interval rates of 0-25 m/m.y. The postplatform sequence shows one distinct hiatus in the interval of 3.3-6.3 mbsf depth that is constrained by nannofossil datums to at least 2.8-1.7 Ma, and possibly up to 3.7-0.9 Ma in age. Peaks in the natural gamma radiation and magnetic susceptibility data at 5.7 ± 0.2 mbsf (see "Core Physical Properties"), and iron-stained sediment observed in Section 194-1193A-1H-4 (see "Site 1193 Visual Core Descriptions") are the sedimentary expression of a hiatus and confine the depth interval of the hiatus further.

Age picks for lithologic and seismic unit boundaries are well defined except for the lithologic Subunit IIIA/IIIB boundary within the carbonate platform and the boundaries below 500 mbsf (Fig. F42; Table T8).

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