Middle Valley

Dead Dog Vent Field

Site 1036

Site 1036 was located at the active Dead Dog vent field (Fig. 4) to (1) constrain the fluid flow rates and pathways for hydrothermal fluid and seawater entrained into the hydrothermal upflow zone, (2) study the mode of formation of the hydrothermal mounds and test a model that predicted the mounds' growth by subseafloor mineral precipitation and inflationary growth, (3) determine the effects of hydrothermal activity on sediment diagenesis and alteration, and (4) determine the presence, continuity, and nature of a suspected cap-rock horizon at 30 mbsf. In order to meet these objectives we drilled Holes 1035A, 1035B, and 1035C along a NW-SE transect from the top to the margin of the 7-m-high Dead Dog active hydrothermal mound. Hole 1036A was located about 9 m west of a 268°C active vent and was cored to 38.5 mbsf. Hole 1036B was offset about 37 m to the NW and cored to a depth of 52.3 mbsf. Hole 1036C was offset another 34 m to the NW and cored to a depth of 54.2 mbsf.


Three major lithologic units identified in the Middle Valley were identified (I, II, and IV), and five subunits defined by alteration were recognized.

Lithologic Unit I is silty clay and has a relatively consistent thickness between holes (25.09 m at Hole 1036A, 25.60 m at Hole 1036B, and 26.70 m at Hole 1036C). The age of this unit is Holocene to late Pleistocene and it is interpreted to be largely a hemipelagic sequence. On the basis of hydrothermal/diagenetic alteration, lithologic Unit I has been subdivided into three subunits.

Subunit IA lacks distinctive hydrothermal or diagenetic products, and consists of slightly altered hemipelagic silty clay with several thin (0.1- to 10-cm-thick) laminae and beds of silt. These silty units are composed of quartz, feldspar, mica, clay, and chlorite with trace amounts of calcite, hornblende, pyroxene, epidote, and pyrite.

Subunit IB is distinguished from Subunit IA by the presence of authigenic carbonate. The core from Hole 1036A contains just a few dolomite nodules. In the core from Hole 1036B, numerous dolomite and calcite nodules are found in the interval from 10.20 to 32.20 mbsf. Hydrothermal alteration also affected the preservation of microfossils in the cores. Core-catcher samples prepared for paleontological analysis were devoid of foraminifers below 9.5 mbsf in Hole 1036A, below 18.6 mbsf in Hole 1036B, and below 35.0 mbsf in Hole 1036C. Siliceous microfossils disappear at even shallower burial depths in all holes.

Subunit IC contains authigenic anhydrite as disseminated crystals, nodules, and cement, and is a distinctive alteration facies of the hydrothermal system. The depth of first appearance of anhydrite was 9.5 mbsf in Hole 1036A and 18.55 mbsf in Hole 1036B. The first appearance of anhydrite in Hole 1036C (Subunit IIB) is at 42.20 mbsf. This depth may indicate the approximate position of the 160°C isotherm.

Lithologic Unit II is characterized by hemipelagic silty clays interbedded with silt to fine sand turbidites. It is subdivided into Subunit IIB, characterized by the occurrence of diagenetic carbonates, and Subunit IIC distinguished by the presence of anhydrite nodules and cement. The boundary between Subunit IIB and IIC is defined as the first down-core appearance of non-vein anhydrite. For example, Hole 1036C contains abundant calcite cement above 40.9 mbsf and contains anhydrite, but no calcite, below 42.0 mbsf.

Lithologic Unit IV. Hole 1036A was drilled less than 10 m from an active hydrothermal vent surrounded by an anhydrite apron formed by collapse of an anhydrite chimney. The first 6.10 m section in Hole 1036A is assigned to Unit IV and accounts for most of the 7-m bathymetric relief of the Dead Dog Mound. Unit IV is a heterogeneous mixture of clasts derived from collapse of anhydrite chimneys, now partly altered to gypsum, very fine-grained pyrrhotite with subordinate pyrite and sphalerite, and greenish gray clay, probably a Mg-bearing smectite. The lowermost 1.6 m of the sequence has a dark color, probably a result of the slow, ongoing process of gypsum dissolution, leaving behind a residue of clay and sulfide. Unit IV is underlain by an unaltered, homogeneous, silty clay that appears to be of hemipelagic origin (Subunit IA).

Drilling at Site 1036 showed that flat lying Subunits IA and IIA have not been significantly displaced since deposition. The relief of the Dead Dog mound is close to the stratigraphic thickness of chimney-derived rubble, hence the mound is nearly entirely a build-up of rubble and is not caused by authigenic hydrothermal mineral precipitation in the subsurface or tectonic uplift. The Dead Dog mound is presumably a recently formed feature. The upper boundaries of the carbonate and anhydrite alteration subunits deepen away from the mound, reflecting the decreasing geothermal gradients.

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