LITHOSTRATIGRAPHY

Site 1141 is located in 1197 m of water near the crest of Broken Ridge. Hole 1141A was rotary cored continuously to a depth of 185.60 mbsf. Sediments were recovered from 0 to 103.80 mbsf. Altered and relatively fresh basalts were recovered from the lower 72.1 m of the hole (Fig. F4; Table T3). The sedimentary section above igneous basement, lithologic Unit I, consists entirely of pelagic sediments. The basement volcanic rocks are designated lithologic Unit II and are subdivided into basement Units 1-6 (Fig. F4, Table T3) (see "Igneous Petrology," "Physical Volcanology," and "Alteration and Weathering" for descriptions). Core recovery in Hole 1141A ranged from excellent to poor (Fig. F4).

Unit I consists predominantly of homogeneous white (N9) foraminifer nannofossil ooze of Pleistocene to middle Miocene age (Fig. F4; Table T3). The CaCO₃ content of Unit I is uniformly high and ranges from 95 to 96 wt% (Table F4). Rare black sand is disseminated throughout Cores 183-1141A-4R and 5R. X-ray diffraction (XRD) analyses show that only calcite is present, except for Section 183-1141A-1R-1, which is ~1% aragonite (Table F4). Drilling disturbance is generally minimal, except for short soupy and slightly disturbed intervals in Cores 183-1141A-1R through 4R.

Core 183-1141A-1R consists of light pale brown nannofossil-bearing foraminifer ooze that is predominantly composed of sand-sized foraminifers and rare shell fragments. Clay- to silt-sized foraminifers (juveniles) and other particles constitute <10% of the sediment. Interval 183-1141A-1R-1, 0 cm, to 2R-2, 26 cm, appears to be a single bed that displays slight normal size grading. Interval 183-1141A-1R-2, 26 cm, to 1R-CC, 16 cm, contains the top of a second bed that appears to display slight normal size-grading. In the top few centimeters of this interval, fine-grained white ooze with abundant nannofossils grades downward into light pale brown ooze composed mostly of sand-sized foraminifers.

The base of Unit I is marked by a layer of sandy nannofossil-bearing foraminifer limestone of late or middle Eocene age. The thickness of this layer is uncertain as only two small fragments were recovered (interval 183-1141A-11R-CC, 0-6 cm). This sediment has an indurated matrix composed of foraminifer tests and lesser amounts of silt- to sand-sized calcareous test fragments and particles. This matrix contains abundant sand- to pebble-sized rock fragments and mineral grains. The sand-sized grains are mainly quartz. Subrounded to well-rounded pebbles, as much as 1 cm in diameter, are composed mainly of chert, silicified sedimentary rock, ferromanganese-encrusted light gray porcellanite, and rare altered basalt. One of the core fragments is partially covered by a ferromanganese crust at least 1 cm thick. Small pockets in this crust are filled with white nannofossil foraminifer ooze of early Miocene age. A thin section of the limestone (Sample 183-1141A-11R-CC, 1-4 cm) shows ~25% foraminifers (mainly planktonic), crinoids (1%), bryozoans (1%), shell fragments (3%), ostracodes (<1%), and sand- to pebble-sized lithic fragments (17%). The pebbles are well-rounded and consist mainly of quartz, microquartz, and volcanic rock. One pebble has phenocrysts. The sand fraction includes quartz and alkali feldspar. Grains are angular. The remainder of the matrix (~45%) mainly consists of micrite and silt-sized bioclasts.

There was no official recovery in Core 183-1141A-12R (cored interval 103.80-113.50 mbsf); however, several 0.25- to 0.75-cm-diameter subrounded pebbles of chert similar to those in Core 183-1141A-11R were in the core catcher along with a tubular-shaped fossil (2 mm in diameter and 3 mm long). Because the original stratigraphic location of these pebbles is uncertain, we can not definitely interpret the composition of this interval; however, we include it as part of Unit I (Fig. F4).

In Hole 1142A, rock fragments of similar lithology to those of Section 183-1141A-11R-CC in Unit I were recovered from interval 183-1142A-1W-1 (Pieces 1 and 2, 0-10 cm). These are light pale brown sandy, pebbly foraminifer limestone with echinoid spines and bivalve(?) shells. Nannofossils indicate an Oligocene or Eocene age for the sediment. Some echinoderm fragments have a syntaxial overgrowth of idiomorphic clear calcite. The surface of a rounded basalt pebble (Piece 5 in Core 183-1142A-1W-1) is coated with sandy packstone containing bryozoans and echinoid spines. A second basalt fragment (Piece 7) has an altered, oxidized surface. A thin layer of pale brown calcareous sandstone with well-sorted, rounded, medium sand is attached to this altered surface. Because these sediments from Hole 1142A were found in a wash core and represent an interval from 0 to 91 mbsf (Fig. F5), the stratigraphic interval from which they were recovered is uncertain.

Lithologic Unit II is composed of basalts, which are highly altered in the upper portion of the section (Fig. F4; Table T3). Unit II is subdivided into 6 basement units, which are described in the "Igneous Petrology," "Physical Volcanology," and "Alteration and Weathering" sections.

The rather homogeneous pelagic sedimentary succession (Unit I) recovered at Site 1141 indicates that Broken Ridge has been at bathyal water depths since at least early Miocene time. The sequence of sediments recovered in Site 1141 is very similar to Unit I of Site 755 (Leg 121, Peirce, Weissel, et al., 1989), also close to the crest of Broken Ridge. In contrast to the sites drilled during Leg 121 on Broken Ridge (Peirce, Weissel, et al., 1989), older sedimentary successions were not recovered between Unit I and basement at Site 1141. Thus, we have no record of the erosional/depositional history of sedimentation prior to early Miocene time. The sediments of lithologic Unit I offer no evidence for the alteration environment (e.g., subaerial or submarine) of the extensively altered basalts of the basement units.

The short interval of sediment at the top (interval 183-1141A-1R-1, 0 cm, to 2R-CC, 16 cm) of Unit I appears to represent a local deposit emplaced by gravity-controlled processes or bottom-current reworking. In the coarse nannofossil-bearing foraminifer ooze, the low abundance of fine particles and normal size-grading suggests deposition by turbidity currents or related gravity-controlled flows. Localized carbonate turbidites and debris-flow deposits are commonly generated by local mass wasting on the sides of seamounts, abyssal hills, and basement ridges, such as the crest of Broken Ridge, which is only a few kilometers from Sites 1141 and 1142. Alternatively, removal of the fine sediment fraction by currents impinging on Broken Ridge may account for this bed.

The nannofossil-bearing foraminifer limestone in the bottom interval (interval 183-1141A-11R-CC, 0-6 cm) with randomly distributed rock and mineral pebbles and sand grains may also represent a debris-flow deposit. However, ferromanganese coatings around pebbles and an in situ ferromanganese crust in the limestone fragments imply a period of very low sedimentation rates and nondeposition, possibly caused by bottom-current activity. Winnowing of fine-grained sediments could explain the high percentage of coarse-grained material. A long period of nondeposition and simultaneous pore-water flushing by bottom currents could also explain the strong cementation of this layer. A possible hiatus of as much as 15 Ma is indicated by the age difference between the limestone (late or middle Eocene) and the overlying ooze (early Miocene).

The occurrence of similar limestone covering a pebble of basalt and similar calcareous sandstone attached to the surface of a basalt fragment in interval 183-1142A-1W, 0-10 cm, suggest that these sedimentary rocks were deposited on top of a basalt layer. Bryozoans, bivalves, and echinoids may indicate shallow marine environments; however, the high abundance of planktonic foraminifers indicates redeposition of these shallow-water organisms to a bathyal environment.