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Title: Silica diagenesis in Santa Cruz mudstone, Late Miocene, California

Abstract

The silica-rich upper Miocene Santa Cruz Mudstone is similar to the Miocene Monterey Formation. Previous studies have suggested the Santa Cruz Mudstone was not buried deeply nor had it undergone extensive diagenesis. Because opaline diagenesis is temperature dependent, the author examined the silica diagenesis of the Santa Cruz Mudstone using scanning electron microscopy and x-ray diffraction methods to study its burial history. In a series of samples from Santa Cruz to Davenport, California (over 16 km), opal-CT is the dominant silica phase present and clay minerals are notably absent. The d(101)-spacing values of opal-CT range from 4.11 A (Santa Cruz area) to 4.06 A (north of Santa Cruz), exhibiting the complete range of d(101)-spacing values found in opal-CT zones. Scanning electron micrographs of crystalline microtextures show rosettes of opal-CT (lepispheres) in cavities of samples with medium to high d(101)-spacing values. The morphology of lepisphere crystallites grades from bladed to spiny with decreasing d(101)-spacing values, reflecting an internal crystal ordering with increased diagenesis. Further diagenetic changes occurred in a sample with 4.06 A d(101)-spacing where incipient quartz crystals signal the initial conversion of opal-CT to microcrystalline quartz. Silica diagenesis demonstrates that burial temperatures surpassed the range of opal-A to opal-CT conversionmore » and approached conversion temperatures (55/sup 0/C to 110/sup 0/C) of opal-CT to microcrystalline quartz. The conversion occurred when the Santa Cruz Mudstone was buried over 1900 m (depth calculated from a geohistory diagram). This burial temperature brings the Santa Cruz Mudstone within the oil generation window, and could account for the presence of hydrocarbons in the unit.« less

Authors:
Publication Date:
Research Org.:
Univ. of California, Santa Cruz
OSTI Identifier:
6255419
Report Number(s):
CONF-870606-
Journal ID: CODEN: AABUD
Resource Type:
Conference
Resource Relation:
Journal Name: AAPG (Am. Assoc. Pet. Geol.) Bull.; (United States); Journal Volume: 71:5; Conference: American Association of Petroleum Geologists annual meeting, Los Angeles, CA, USA, 7 Jun 1987
Country of Publication:
United States
Language:
English
Subject:
02 PETROLEUM; CALIFORNIA; PETROLEUM DEPOSITS; SILTSTONES; RESERVOIR ROCK; DIAGENESIS; GEOLOGIC FORMATIONS; TERTIARY PERIOD; CENOZOIC ERA; FEDERAL REGION IX; GEOLOGIC AGES; GEOLOGIC DEPOSITS; MINERAL RESOURCES; NORTH AMERICA; RESOURCES; ROCKS; SEDIMENTARY ROCKS; USA 020200* -- Petroleum-- Reserves, Geology, & Exploration

Citation Formats

El-Sabbagh, D. Silica diagenesis in Santa Cruz mudstone, Late Miocene, California. United States: N. p., 1987. Web.
El-Sabbagh, D. Silica diagenesis in Santa Cruz mudstone, Late Miocene, California. United States.
El-Sabbagh, D. 1987. "Silica diagenesis in Santa Cruz mudstone, Late Miocene, California". United States. doi:.
@article{osti_6255419,
title = {Silica diagenesis in Santa Cruz mudstone, Late Miocene, California},
author = {El-Sabbagh, D.},
abstractNote = {The silica-rich upper Miocene Santa Cruz Mudstone is similar to the Miocene Monterey Formation. Previous studies have suggested the Santa Cruz Mudstone was not buried deeply nor had it undergone extensive diagenesis. Because opaline diagenesis is temperature dependent, the author examined the silica diagenesis of the Santa Cruz Mudstone using scanning electron microscopy and x-ray diffraction methods to study its burial history. In a series of samples from Santa Cruz to Davenport, California (over 16 km), opal-CT is the dominant silica phase present and clay minerals are notably absent. The d(101)-spacing values of opal-CT range from 4.11 A (Santa Cruz area) to 4.06 A (north of Santa Cruz), exhibiting the complete range of d(101)-spacing values found in opal-CT zones. Scanning electron micrographs of crystalline microtextures show rosettes of opal-CT (lepispheres) in cavities of samples with medium to high d(101)-spacing values. The morphology of lepisphere crystallites grades from bladed to spiny with decreasing d(101)-spacing values, reflecting an internal crystal ordering with increased diagenesis. Further diagenetic changes occurred in a sample with 4.06 A d(101)-spacing where incipient quartz crystals signal the initial conversion of opal-CT to microcrystalline quartz. Silica diagenesis demonstrates that burial temperatures surpassed the range of opal-A to opal-CT conversion and approached conversion temperatures (55/sup 0/C to 110/sup 0/C) of opal-CT to microcrystalline quartz. The conversion occurred when the Santa Cruz Mudstone was buried over 1900 m (depth calculated from a geohistory diagram). This burial temperature brings the Santa Cruz Mudstone within the oil generation window, and could account for the presence of hydrocarbons in the unit.},
doi = {},
journal = {AAPG (Am. Assoc. Pet. Geol.) Bull.; (United States)},
number = ,
volume = 71:5,
place = {United States},
year = 1987,
month = 5
}

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  • The upper-Miocene-lower Pliocene laminated to massive diatomaceous strata of the Santa Maria basin encompass portions of the Monterey and overlying conformable to nonconformable Sisquoc Formations and record the continued accumulation of admixed siliceous (mostly diatoms) and terrigenous (silt and clay) components in a Neogene borderland basin. Lithologies, which vary from diatomite to diatomaceous mudstone, are the result of fluctuations in slope-accumulating silica and terrigenous debris. Bulk accumulation rates, as well as silica and terrigenous component accumulation rates, were calculated for four measured sections using accurate stratal thicknesses, absolute ages, rock compositions, and bulk densities. Bulk accumulation rates range from lessmore » than 20 to greater than 70 mg/cm/sup 2//year and are comparable to bulk accumulation rates in the present-day Santa Barbara basin and Gulf o f California. Silica accumulation rates (20-30 mg/cm/sup 2//year) suggest that sedimentation took place beneath productive waters and they exceed those encountered in other high-productivity oceanic areas, such as the Bering Sea and Peruvian coastal waters. Relatively high terrigenous accumulation rates (18-35 mg/cm/sup 2//year) reflect the dilution of these high-productivity waters by continentally derived detritus. Comparison of calculated accumulation rates with rates determined for similarly aged strata of the Santa Barbara basin reveals that silica and terrigenous debris were accumulating at higher rates within the Santa Maria basin. These differences are due to the complex interplay of tectonics and oceanographics and to the proximity of the basins relative to the strandline.« less
  • Geochemical studies of volcanic clasts from the middle Miocene Blanca Formation exposed on Santa Cruz and Santa Rosa Islands confirm earlier studies that suggested correlation of these two occurrences on the basis of stratigraphic position and lithology. Clasts from both areas are calc-alkaline dacite and rhyolite that define smooth trends on element-variation diagrams and possess unusually low incompatible trace-element concentrations and Sr-isotopic ratios when compared to other typical calc-alkaline volcanic suites: Rb ranges from 6 to 60 ppm, Sr from 260 to 575 ppm, and {sup 87}Sr/{sup 86}Sr averages 0.7033. The main exposure of this formation on Santa Cruz Islandmore » has been divided into three members. Based on elemental similarities, clasts from the Santa Cruz Near Point exposure are most similar to those from the lower two members, whereas clasts from the Santa Rose exposure are more similar to those from the upper member. Clasts from all three exposures likely were derived from the same volcanic complex before being deposited in a single submarine-fan complex. We conclude that these rocks were erupted and deposited onto the Baja Borderland allochthon by subduction-related volcanism that was active south of the Rivera triple junction. The fan complex was then translated on this allochthon northward relative to the North American craton and rotated {approximately} 90 to 128{degree}CW to its present position.« less
  • The Santa Cruz (La Honda) Basin is a small [open quote]slice[close quote] of the San Joaquin Basin that has been displaced c. 300 km to the northwest by the San Andreas Fault. The poorly-explored offshore area that now lies within the Monterey Bay NMS includes portions of the Outer Santa Cruz and Bodega basins. A modest amount (c. 1.3 MM bbl) of variable-quality oil has been produced from Eocene and Pliocene pay zones in the La Honda Field. Much smaller amounts of light oil ([ge]40[degrees] API) have been produced from three other fields (Oil Creek; Moody Gulch; Half Moon Bay).more » Large tar deposits also outcrop near the city of Santa Cruz. Proven source rocks in this basin include the Eocene Twobar Shale and three Miocene units: the Lambert Shale, Monterey Formation, and the Santa Cruz Mudstone. A high-gravity oil sample from the Oil Creek Field contains isotopically-light carbon ([delta][sup 13]C = - 28.2 per mil) and has a relatively high pristane/phytane ratio. This oil was generated at high temperature (c. 140[degrees]C) by pre-Miocene source rocks (probably the Twobar Shale). The presence of isotopically-heavy carbon in all other oil and tar samples demonstrates they were generated by Miocene source rocks. But the C[sub 7] oil-generation temperatures, sulfur content, vanadium/nickel ratios, and biomarker chemistry of these Miocene oils are significantly different than in Monterey oils from the prolific Santa Maria Basin (SMB). The sulfur content (8.0 wt%) and V-Ni chemistry of tarry petroleum recovered in the P-036-1 well (Outer Santa Cruz Basin) resembles the chemistry of very heavy (<15[degrees]API) oils generated by phosphatic Monterey shales in the SMB.« less
  • The Santa Cruz (La Honda) Basin is a small {open_quote}slice{close_quote} of the San Joaquin Basin that has been displaced c. 300 km to the northwest by the San Andreas Fault. The poorly-explored offshore area that now lies within the Monterey Bay NMS includes portions of the Outer Santa Cruz and Bodega basins. A modest amount (c. 1.3 MM bbl) of variable-quality oil has been produced from Eocene and Pliocene pay zones in the La Honda Field. Much smaller amounts of light oil ({ge}40{degrees} API) have been produced from three other fields (Oil Creek; Moody Gulch; Half Moon Bay). Large tarmore » deposits also outcrop near the city of Santa Cruz. Proven source rocks in this basin include the Eocene Twobar Shale and three Miocene units: the Lambert Shale, Monterey Formation, and the Santa Cruz Mudstone. A high-gravity oil sample from the Oil Creek Field contains isotopically-light carbon ({delta}{sup 13}C = - 28.2 per mil) and has a relatively high pristane/phytane ratio. This oil was generated at high temperature (c. 140{degrees}C) by pre-Miocene source rocks (probably the Twobar Shale). The presence of isotopically-heavy carbon in all other oil and tar samples demonstrates they were generated by Miocene source rocks. But the C{sub 7} oil-generation temperatures, sulfur content, vanadium/nickel ratios, and biomarker chemistry of these Miocene oils are significantly different than in Monterey oils from the prolific Santa Maria Basin (SMB). The sulfur content (8.0 wt%) and V-Ni chemistry of tarry petroleum recovered in the P-036-1 well (Outer Santa Cruz Basin) resembles the chemistry of very heavy (<15{degrees}API) oils generated by phosphatic Monterey shales in the SMB.« less
  • A comparative study of authigenic carbonates formed in five organic-rich, deep-water units of the La Honda basin indicates that sedimentation rate and degree of bottom-water oxygenation ultimately controlled the timing, chemical environment, mineralogy, and rate of authigenic carbonate formation. Additionally, sedimentation rate and oxygenation of bottom water governed the sequence of diagenetic chemical environments, the degree of preservation of organic matter, and the source bed potential of the basinal units. The conditions necessary for the preservation of organic matter and the development of potentially good source rocks were variable. However, the occurrence of anaerobic bottom waters was the critical elementmore » in the development of excellent source rocks (i.e., Twobar Shale Member, Lambert Shale) regardless of the rate of sedimentation or the rate of primary productivity. In contrast, in basinal units characterized by oxygenated bottom waters, high rates of sedimentation, and high rates of primary productivity (Monterey Formation and Santa Cruz Mudstone Member), rapid burial of organic matter promoted highly anaerobic diagenetic environments which decreased the rate of organic matter decomposition, thereby producing units with good potential source rocks.« less