Organic geochemistry and clay mineralogy of Lower Cretaceous sediments from Allison and Resolution Guyots (Sites 865 and 866), Mid-Pacific Mountains

Clay- and organic-rich facies were deposited in Early Cretaceous shallow-water environments from both Allison (Hole 865A)
and Resolution (Hole 866A) guyots in the northwestern Pacific Ocean. Together with mineralogical, sedimentological, and
petrographical examinations, organic matter was characterized using Rock-Eval pyrolysis, vitrinite reflectance measurements,
elemental analysis of selected kerogens, and chromatography of the saturated and unsaturated hydrocarbons.
Low to very low organic carbon content characterizes gray green pyritic clay stones and most of the bioturbated clayey wackestones and packstones in Hole 865A, whereas organic carbon contents up to 50% are recorded in black and brown claystones.
Percentages of organic carbon in Hole 866A present high-amplitude variations that range from 0.1 % to 34.5%. As a general rule,
total organic carbon contents higher than 3% are recorded in laminated limestones, algal mats, and claystones with plant debris,
whereas gray limestones, sandy claystones, and green claystones have organic carbon contents lower than 0.5%.
According to the hydrogen indices, H/C ratios of kerogen concentrates, and the richness in vitrinite, the origin of organic matter
is mainly terrestrial (type III) in Hole 865 A. At the base of this hole, two organic-rich samples with well-preserved algal structures,
high hydrogen indices, and high H/C ratios contain type I or II organic matter. In Hole 866A, the organic matter is widely distributed
between type I (algal or bacterial) and type IV (altered organic matter), according to the wide range of hydrogen indices. Laminated
limestones and algal mats are related to type I or II organic matter, according to their high hydrogen indices and H/C ratios, as well
as their richness in lamalginite and bituminite. Black claystones rich in vitrinite are related to type III (terrestrial), whereas other
lithologies generally contain a type IV (oxidized) organic matter.
Tmax and vitrinite reflectance measurements, as well as biomarker abundance, suggest that the organic matter is too immature
for hydrocarbon generation.
The composition of clay-mineral assemblages results from various early diagenetic, volcanic, and detrital influences that
reflect the evolution of the Lower Cretaceous sedimentary environments of Allison and Resolution guyots.
Most of the sulfur in the sediment is in the form of pyrite at Hole 865A, whereas the proportion of sulfur in organic form is
much more important in sediments from Hole 866A, where cyanobacterial mats predominate.
Variations in the quantity and quality of clay minerals and organic matter are strongly dependent on the lithology, as well as
the paleoenvironmental evolution of the guyots. During the early history of Allison Guyot, soil-derived material, including
illite-smectite mixed-layers, kaolinite, and terrestrial organic matter, were probably supplied from residual volcanic islands. This
land-derived influx became less abundant upward and progressively disappeared, suggesting that the volcanic islands were
definitively submerged. The clay- and marine organic-rich facies were mainly deposited during lagoonal to peritidal phases of the
history of Resolution Guyot. Nevertheless, terrestrial organic matter occurs here and there, implying the existence of nearby
vegetated islands.
Comparison of Aptian sediments from Hole 866A with those of DSDP Site 463 suggests that the late early Aptian Oceanic
Anoxic Event, corresponding to the Selli level, may be recorded at both sites.