X‐ray diffraction and oxygen isotopic analyses of outcrop and subsurface samples of siliceous rocks were used to reconstruct thermal and diagenetic histories of the Miocene Monterey Shale near Santa Maria, California. Within many stratigraphic sections soft, porous diatomaceous rocks change gradationally to underlying hard and brittle chert, porcellanite, and siliceous shale; the accompanying silica mineral zones are, in descending stratigraphic order: (1) biogenic silica (opal‐A), (2) cristobalitic silica (opal‐CT), and (3) microcrystalline quartz.
Boundaries between silica mineral zones and stratigraphic horizons are often discordant. Within the opal‐CT zone, the d(101)‐spacing of opal‐CT decreases in a smooth non‐linear fashion from about 4 10 Å to 4‐04 Å. In the Santa Maria Valley and Bradley oil field areas the thicknesses of the opal‐CT zones are greater and the present thermal gradients less than in the adjacent Orcutt oil field. Thin opal‐CT zones at shallow maximum burial depths apparently correlate with higher thermal gradients.
Using present thermal gradients and reconstructed maximum burial depths from well data in the Santa Maria region, the ranges in temperatures for the top and base of the opal‐CT zone are 38–54 °C and 55–110 °C, respectively. The temperature difference between these two boundaries ranges from 17 to 60 °C. In comparison, temperature ranges for these two boundaries computed from oxygen isotopic compositions of opal‐CT and quartz, extrapolated experimental quartz‐water fractionations, and assuming δO18= 0%o for the isotopic composition of the equilibrating fluid are 18–56 °C and 31–80 °C for the top and base of the opal‐CT zone, respectively. The temperature difference between these boundaries is 11–36 °C using this method.
Thermal gradients and sedimentation rates strongly influence rates of silica transformations. Reconstructed thermal and diagenetic histories of siliceous rocks of the Monterey Shale at four well sites in the Santa Maria region demonstrate that most silica conversions probably occurred during the last 3–4 Myr in response to accelerated rates of sedimentation (and therefore burial heating) during the Pliocene.
The latest Mesozoic and earliest Tertiary sediments at Deep Sea Drilling Project site 524 provide an amplified record of environmental and biostratographic changes at the end of Cretaceous. Closely spaced samples, representing time intervals as short as 10(2) or 10(3) years, were analyzed for their bulk carbonate and trace-metal compositions, and for oxygen and carbon isotopic compositions. The data indicate that at the end of Cretaceous, when a high proportion of the ocean's planktic organisms were eliminated, an associated reduction in productivity led to a partial transfer of dissolved carbon dioxide from the oceans to the atmosphere. This resulted in a large increase of the atmospheric carbon dioxide during the next 50,000 years, which is believed to have caused a temperature rise revealed by the oxygen-isotope data. The lowermost Tertiary sediments at site 524 include fossils with Cretaceous affinities, which may include both reworked individuals and some forms that survived for a while after the catastrophe. Our data indicate that many of the Cretaceous pelagic organisms became extinct over a period of a few tens of thousands of years, and do not contradict the scenario of cometary impact as a cause of mass mortality in the oceans, as suggested by an iridium anomaly at the Cretaceous-Tertiary boundary.
One of the important results of the Japan Sea drilling by ODP is the detection of an opal-A/opal-CT transformation at all the drill sites. The transformation boundary is well recognized in physical property and downhole measurements as well as in lithologic descriptions. Because the boundary shows a marked change of density, the boundary appears in seismic reflection profiles as a BSR. The transformation of opal-A to opal-CT is closely correlated with temperature and age of host rock in the Japan Sea. We estimate and evaluate Japan Sea heat flow by using the opal-A/opal-CT bottom simulating reflector (BSR) observed in seismic reflection profiles. Then we estimated the age of host rock from available seismic profiles based on the assumption of constant sedimentation rate. Therefore, the temperatures at opal-A/opal-CT boundary were estimated from seismic profiles. The temperature data were converted to surface heat flow data by using the thermal conductivity data obtained by shipboard physical property measurements. The estimated BSR heat flow showed significant agreement with individual probe heat flow data. We confirmed the validity of BSR heat flow on a multichannel seismic profile. This method can provide large areal estimates of the surface heat flow. The opal-A/opal-CT BSR can contribute to defining the thermal structure of the Yamato Basin, Japan Sea.
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