Sites 535, 539, and 540

Abstract

SITE 539 (HOLE 539) SUMMARYSites 535, 539, and 540 were drilled in the eastern part of the study area (Fig. 1). They all had the same major objective, i.e., to examine an inferred mid-Cretaceous unconformity (MCU) and to penetrate as deep as possible into the thick pre-mid-Cretaceous sedimentary basin below. The sites have been designated the "basin sites." The pre-mid-Cretaceous rocks either crop out or subcrop beneath a thin Cenozoic cover along the eastern flank of a broad north-south erosional channel (Fig. 1). Sites 535 and 540 were positioned along this erosional slope, and together, they quite successfully sampled an almost continuous section of Lower Cretaceous limestones, plus the thin overlying Cenozoic cover. Seismic Line SF-15 shows the overall setting of the two sites (Fig. 2). Holes 539 and 539A represent unsuccessful attempts to spud.Because of their similar locations and objectives, discussion of the three sites are combined into this one overall site chapter. The major results of Sites 535 and 540 are shown on summary diagrams (Figs. 3 and 4, respectively) and are discussed further below.Sites 535 and 540 together penetrated over 1400 m of Cenozoic and Cretaceous section, with Hole 540 representing the upper part. The major lithologic sequences in Hole 540 from top to bottom are as follows:1. The top consists of 272 m of carbonate ooze, chalk, and marly limestone with ash layers; the lower part is deformed by creep and slumping; age ranges from Pleistocene to late Paleocene.2. In the middle are gravity-flow deposits (56 m)-mainly layered chalk and pebbly chalks and limestone; pebbles consist of shallow-water and deep-water limestone clasts; this interval includes a 3-4-m graded sandstone-conglomerate toward the top consisting of altered volcanic and limestone material; age ranges from late Paleocene to middle Cenomanian.3. The lower section contains 417 m of limestone with rhythmic alternations of bioturbated, light-colored layers and laminated, dark, organic matter-rich layers; thin layers of carbonate silt and sand occur throughout; black chert is conspicuous in upper half; there are intercalations of several meters of homogeneous, micritic limestones and calcarenites with shallow-water detritus at the base; age ranges from lower Cenomanian to middle Albian.The major lithologic sequences in Hole 535 are as follows:1. The top contains 154 m of mainly upper Pleistocene clay and mud.2. In the middle 233 m, banded, laminated, and variably bioturbated limestone occurs with layers of coarse skeletal debris, particularly toward the top; age is controversial-a late Albian(?)-Cenomanian(?) age is based on ammonites.3. The lower section consists of 327 m of rhythmic alternations of (1) light-colored, massive, and variably bioturbated limestones; (2) darker laminated limestones, and (3) very dark carbonaceous, marly limestones with fine laminations; hardgrounds and cephalopod limestones are common toward the bottom; age ranges from Albian(?) to late Berriasian.These two holes together provide the first v...