165 Preliminary Report

Site 1001 Results

Site 1001 is located in an area of the lower Nicaraguan Rise where the Neogene sediments are thin, and where a continuous Upper Cretaceous-Paleogene sedimentary sequence overlying basaltic basement was expected. The interval between the widespread Caribbean seismic reflectors A" (lower Eocene cherts) and B" (Upper Cretaceous basalt) had been cored but poorly recovered at DSDP Site 152, located ~40 km to the east-northeast, at the base of the Hess Escarpment. At 3270 m water depth, Site 1001 is 600 m shallower than Site 152, and is located on the Hess Escarpment. The principal objectives at Site 1001 were to (1) recover a complete Cretaceous/Tertiary boundary sequence, (2) address topics of ancient ocean paleoceanography, including the tropical record of the late Paleocene thermal maximum and the nature of orbital forcing during the Maastrichtian and Paleocene, and (3) recover igneous basement and test models of the formation of the Caribbean Oceanic Plateau. Most of the Paleogene and Cretaceous section was double-cored including the conformable basalt/limestone contact. Basalt of probable mid-Campanian age (~77 Ma) was penetrated at 485.4 mbsf, and 37.4 m of the basalt was cored.

A 165.7 m Neogene section, spanning the middle Miocene to Pleistocene, is separated from the underlying Paleogene-Cretaceous section by a pair of unconformities in Section 165-1001A-18R-4. Middle Miocene nannofossil ooze unconformably overlies middle Eocene chalk, but the middle Eocene chalk is only 28 cm thick. A second unconformable contact is marked by a chert layer where the thin middle Eocene chalk overlies lower Eocene chalk. The duration of these hiatuses is approximately 30 m.y. and 8 m.y., respectively.

Four lithologic units are recognized at this site (Fig. 13). Unit I is subdivided into four subunits: Subunit IA (6.4-112.2 mbsf; Pleistocene-upper Miocene) consists of clayey nannofossil mixed sediment to nannofossil ooze with clay. Subunit IB (112.2-131.7 mbsf; upper Miocene) is a clayey nannofossil ooze with foraminifers and is marked by a sharp increase in carbonate content and increased induration. Subunit IC (131.7-153.2 mbsf; upper Miocene-middle Miocene) is composed of nannofossil ooze with clay to nannofossil clay and is distinguished from the intervals above and below by the highly variable carbonate contents and magnetic susceptibility. Subunit ID (153.2-165.7 mbsf; middle Miocene) contains nannofossil ooze with clay to clayey nannofossil ooze; this interval is distinctly more carbonate-rich than Subunit IC and has much lower and less variable values of magnetic susceptibility.

Lithologic Unit II corresponds with the Paleocene-Eocene section. The distribution of chert is the principal feature used to subdivide Unit II into two subunits. Subunit IIA (165.7-304.6 mbsf; middle and lower Eocene-upper Paleocene) is primarily composed of calcareous chalk with foraminifers to mixed sedimentary rock with clay. This subunit is interbedded with numerous chert and volcanic ash layers. Subunit IIB (304.6-352.1 mbsf; upper Paleocene-lower Paleocene) lacks the cherts of Subunit IIA. It is more clay-rich, especially in the lower Paleocene, and is further distinguished by the presence of thin interbedded foraminiferal-rich sand layers. The dominant lithologies of Subunit IIB are calcareous chalk with clay to claystone and some ash layers.

The Cretaceous/Tertiary boundary interval was recovered in both Hole 1001A and Hole 1001B (Sections 165-1001A-38R-CC and 39R-1, and Section 165-1001B-18R-5). Comparison of the Formation MicroScanner (FMS) data and the recovered sediments indicates that 15-20 cm of the boundary deposit may not have been recovered (Fig. 14). Remarkably, however, several clay-rich units between the basal Paleocene and upper Maastrichtian limestones were recovered. A 1.7-4.0 cm light gray, highly indurated limestone of earliest Paleocene age (planktonic foraminifer Zone P0/Pa, undifferentiated), similar to the limestone recovered earlier at Site 999, overlies the package of clay-rich strata constituting the bulk of the recovered boundary deposit at Site 1001. The topmost layer of the boundary deposit is a 3.5-cm-thick, massive clay. This unit contains rare grains of shocked quartz and overlies a 3.5-cm-thick smectitic claystone with dark green spherules. The spherules are up to 2 mm in diameter, and may represent altered tektites from the K/T impact event. The base of the boundary deposit is a 1-to 2-cm-thick smectitic clay layer with shaley cleavage. This clay contains light-colored speckles, up to 1 mm in diameter. In addition to these three distinctive clay layers, two loose pieces of polymict micro-breccia were recovered from the top of Core 165-1001A-39R. These contain angular clasts (<6 mm) of claystone and limestone in an unconsolidated matrix of smectitic clay. This lithology may represent fragments of a thicker poorly recovered unit at this site. The total boundary deposit has an inferred thickness of approximately 25 cm at this location.

The Upper Cretaceous sedimentary section is represented by lithologic Unit III, which is subdivided into two subunits. A marked increase in carbonate content delimits the change from lower Paleocene mixed sedimentary rocks and claystones to Maastrichtian limestone. Subunit IIIA (352.1-472.9 mbsf; basal Paleocene to mid-Campanian) consists of calcareous limestone and claystone with interbedded foraminiferal-rich sand layers. Ash layers become thicker and more frequent in the lower part of Subunit IIIA. Subunit IIIB (472.9-485.4 mbsf; mid-Campanian) is characterized by a significant reduction in carbonate content and a dramatic increase in the abundance of altered volcaniclastic material, including common andesitic to silicic ash fall layers and several thick ash turbidites. The lower part of this unit contains angular to subangular fragments of basaltic lapilli and hyaloclastite breccia that grade downcore into a conformable basement contact consisting of sediment-poor basaltic lapilli and basalt.

Lithologic Unit IV (485.4-522.8 mbsf; mid-Campanian), the igneous basement, consists of a succession of 12 formations, which likely represent individual pillow lavas and sheet flows. Some of the flows have thick hyaloclastite breccia tops and massive columnar interiors.

Several lines of evidence support the hypothesis that the volcanic edifice cored at Site 1001 subsided rapidly in mid-Campanian time. Vesicles in the basalt are relatively large, suggesting water depths significantly shallower than the present. Benthic foraminifers from limestone lenses between the basalt flows suggest neritic paleodepths, whereas a rapidly deepening upward trend is suggested on the basis of benthic foraminifer assemblages in the overlying limestones and ash turbidites. The volcanic edifice was likely located near the paleoequator as suggested by the very shallow paleomagnetic inclinations obtained from the basalt and overlying sediments.

A carbonate-poor, clay-rich interval in the uppermost Paleocene is similar to a correlative interval cored earlier at Site 999. Both sites contain volcanic ash layers within the distinctive laminated to weakly bioturbated deposit. We attribute the character of this interval to the rapid and short-lived oceanographic changes of the "late Paleocene thermal maximum." The interval was cored and recovered twice at Site 1001 (Sections 165-1001A-27R-2 and 165-1001B-6R-3).

Ash layers representing three episodes of volcanism in the Caribbean were found in the Paleogene-Upper Cretaceous of Site 1001 (Fig. 4). One episode in the latest Paleocene-early Eocene time is likely related to the explosive volcanism documented at Site 998 and attributed to the Cayman Ridge arc. A second smaller peak of volcanism occurred in the early Paleocene, and it is perhaps contemporaneous with the activity recorded at Site 999 on the Kogi Rise and attributed to the Central American arc. A third short-lived episode occurred in mid-Campanian time, perhaps associated with the activity of central volcanoes on the Caribbean Oceanic Plateau.

The middle/upper Miocene boundary interval at Site 1001 is distinguished by highly variable carbonate contents and magnetic susceptibility, and correlates with the "carbonate crash" interval recognized at Sites 998, 999, and 1000. However, in contrast with these earlier sites, the interval of reduced carbonate values persists about one million years longer at Site 1001.

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