165 Preliminary Report

Site 999 Results

Site 999 is located on the Kogi Rise, a previously unnamed bathymetric high in the Colombian Basin. Situated northeast of Mono Rise and southeast of the Hess Escarpment, the crest of the Kogi Rise lies some 1000 m above the turbidite-laden floor of the Colombian Basin and is isolated by a saddle from sediments originating from the Hess Escarpment. A continuous Upper Cretaceous to Holocene record was anticipated. This section was targeted for tropical paleoceanographic studies including the closure of the Central American Seaway in the mid-Pliocene, as well as an undisturbed Cretaceous/Tertiary (K/T) boundary sequence located relatively proximal to the Chicxulub impact site. With the eventual goal of coring basement and testing the hypothesis of a hot spot origin for the Caribbean oceanic plateau (i.e., a Large Igneous Province), Site 999 was designated a 'legacy site' and a reentry cone and casing were installed in Hole 999B for future reoccupation of the site.

A continuous and apparently complete upper Maastrichtian-Pleistocene section was cored at Site 999 (Fig. 7). Hole 999A was APC cored to a depth of 197.6 mbsf (upper Miocene) with 104.6% recovery, and then XCB cored to a depth of 566.1 mbsf with 89.0% recovery. Hole 999A terminated in lower Miocene clayey calcareous chalk. A reentry cone and 62 m of 16-in. casing were jetted into the seafloor before Hole 999B was drilled ahead and casing installed to a depth of 524 m. Coring operations resumed with the RCB at a depth of 543.4 mbsf and terminated in upper Maastrichtian limestone with clay at a depth of 1066.4 mbsf. Recovery with the RCB was good, with 76.1% of the drilled interval recovered.

Mixtures of biogenic ooze, nepheloid clays, and volcanic ash are the primary constituents of the sediments on Kogi Rise. Six lithologic units were recognized at Site 999. Unit I is subdivided into three subunits. Subunit IA (0.0-150.1 mbsf; Pleistocene-lower Pliocene) consists of nannofossil and foraminiferal clayey mixed sediment with scattered interbedded ash layers. Subunit IB (150.1-229.5 mbsf; lower Pliocene-upper Miocene) is similar to Subunit Ia but contains fewer foraminifers, and Subunit IC (229.5-265.1 mbsf; upper Miocene) is further distinguished by the presence of siliceous microfossils. Unit II is subdivided into two subunits: Subunit IIA (265.1-301.8 mbsf; middle/upper Miocene boundary interval) consists of interbedded clay with nannofossils, clayey nannofossil mixed sediment, and common ash layers, and Subunit IIB (301.8-346.9 mbsf; middle Miocene) is similar to Subunit IIA but contains siliceous microfossils and a higher carbonate content. Unit III (346.9-566.1 mbsf; middle to lower Miocene) consists of clayey calcareous chalk with foraminifers and nannofossils, interbedded with abundant ash layers. Unit IV is subdivided into four subunits. Subunit IVA (572.6-644.9 mbsf; lower Miocene-upper Oligocene) consists of clayey calcareous limestone with common thin ash layers, and Subunit IVB (644.9-690.4 mbsf; upper to lower Oligocene) is similar to Subunit IVA but contains more clay. Subunit IVC (690.4-866.2 mbsf; lower Oligocene-middle Eocene) is also a clayey calcareous limestone but contains thicker and more frequent ash layers than Subunits IVA and IVB. Subunit IVD (866.2-887.0 mbsf; middle Eocene) is likewise similar to Subunit IVb but contains more clay. Unit V is subdivided into two subunits: Subunit VA (887.0-1033.4 mbsf; middle Eocene-upper Paleocene) is a clayey calcareous mixed sedimentary rock with some interbedded ash layers, and Subunit VB (1033.4-1049.3 mbsf; lower Paleocene) is a claystone with some interbedded ash layers. Unit VI (1049.3-1066.4 mbsf) consists of basal Paleocene hard light gray limestone and upper Maastrichtian limestone with clay (Fig. 7).

Shipboard biostratigraphy and magnetostratigraphy suggest that the cored section is complete, although the abrupt changes in lithology at the base of the Paleocene may indicate that some of the bedding surfaces recovered in the Cretaceous/Tertiary boundary interval are not conformable or that recovery was incomplete. A comparison of logging records and the cored sediments suggests that a nearly complete boundary sequence was recovered (Fig. 8). The position of the boundary has been constrained by shipboard paleontology to within 10 cm, and extensive shore-based research is planned. The K/T boundary is tentatively placed at the base of a limestone in Section 165-999B-60R-1, 10 cm (1050.2 mbsf).

The middle to upper Eocene and the lower to middle Miocene portions of the section include an unexpectedly large number of rhyolitic volcanic ash layers (>1200), most likely derived from distant silicic volcanic centers in Central America. These ash layers define two major explosive volcanic episodes, also recorded in the sediments at Site 998 on the Cayman Rise (Fig. 4). Volcanic ash layers account for over 4% of the recovered succession. In addition, solid phase geochemistry reveals that the dispersed ash comproses ~18% of the total sediment recovered at Site 999, and that significant amounts of clays in the background sediment are the altered products of volcanic glass (Fig. 5).

The terrigenous component of the sediment has been quantified through an analysis of chromium variation in the bulk sediment. The composition and quantity of this terrigenous sediment may be closely linked to the history of the Magdalena Fan, which has grown in response to the uplift and erosion of the Andean Cordillera. Growth of the submarine fan has been particularly active since the late Miocene, as evidenced by a marked increase in terrigenous mass accumulation rates (1-2 g/cm2/k.y.) and an increase in detrital clays (Fig. 5 and Fig. 9).

Below the middle Eocene ash layers is a series of radiolarian and foraminifer-rich layers, caused by winnowing on the seafloor or by productivity events in the upper water column. In either case, changes in oceanic circulation between the Caribbean and eastern Pacific are suspected, perhaps related to tectonically influenced portals or to sources of deep/intermediate water masses during late Paleocene-early Eocene time. An anomalous band of dark laminated claystone in the uppermost Paleocene of Site 999 may represent an expanded tropical record of the widespread oceanographic warming event that occurred during latest Paleocene time.

The transition from the middle to upper Miocene is distinguished by a sharp reduction in carbonate content and a marked increase in magnetic susceptibility and in terrigenous mass accumulation rates. This interval is correlative to the late Miocene "carbonate crash" of the central and eastern equatorial Pacific, and is also recognized at Site 998 on the Cayman Rise (Fig. 10). Originally thought to be related to tectonically influenced changes in deep water exchange between the Pacific and Atlantic, linkages with changes in thermohaline circulation may be responsible for the widespread "carbonate crash" on both sides of the present-day Isthmus of Panama.

Preliminary evidence for oceanographic changes resulting from the late Neogene closure of the Central American Seaway is found in the planktonic foraminifer record at Site 999. Sinistrally coiled Neogloboquadrina pachyderma, normally a polar/subpolar species, is found in significant numbers through the upper Miocene-lower Pliocene interval. The termination of this interval is linked to the cessation of regional upwelling in the southwestern Caribbean with the closure of the seaway.

The upper Maastrichtian through Oligocene section accumulated at rates of approximately 9-14 m/m.y. (2.2-3.6 g/cm2/k.y.), although the lower Paleocene is characterized by sedimentation rates that average <5 m/m.y (1.2-1.6 g/cm2/k.y.). A marked rise in sedimentation rates and bulk mass accumulation rates through the lower Miocene (29 m/m.y.; 3.5-6.5 g/cm2/k.y.) is due in part to the voluminous volcanic ash input. This interval is followed by reduced rates in the middle and upper Miocene (19 m/m.y.; 1.5-2.5 g/cm2/k.y.), an interval that includes the "carbonate crash." The Pliocene-Pleistocene interval is characterized by a return to higher sedimentation and mass accumulation rates (30-33 m/m.y.; 2.6-3.4 g/cm2/k.y.) in response to increased terrigenous input from the northern Andes.

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