Diatomaceous Rocks Research Articles

Effects of the time-dependent behavior on strain softening of diatomaceous soft rock

This paper is concerned with a new concept of modeling the strain softening behavior of diatomaceous soft rocks based on the laboratory investigation. An approximate solution was described for a stress-strain-time relation taking into account strain softening phenomenon. The treatment of the post-peak behavior in the calculation is discussed and comparisons of the experimental observations of normally consolidated specimens with results of numerically simulated undrained triaxial tests are given. The strain rate effect and strain softening behaviors are controlled by the time dependent stress strain relation.

Seismic Expressions of Monterey Formation Diagenesis: Examples from Offshore California: ABSTRACT

Diagenesis of the diatomaceous rocks in the Monterey Formation in California coastal and offshore basins involves changes from amorphous biogenic silica to a stable crystalline quartz facies. In the intermediate stage, the transformation undergoes passage from the Opal-A to the Opal-CT phase. Associated with this diagenetic process is a marked increase in bulk densities between the different silica phases, owing to loss of porosity from compaction and solution recrystallization caused by increase in burial load and other physical factors. The sharp density contrast between the silica phases is manifested by an acoustic impedance boundary that may be expressed on seismic records. This seismic event can be distinct and independent of structural configuration, and in many places cuts through stratigraphic boundaries. Several examples of seismic records from offshore California demonstrate the diagenetically caused reflection cutting through Monterey and post-Monterey formations. Current and future exploration efforts in offshore California will continue to center on the widespread Monterey Formation. In addition to being the main source rock, the Monterey is also the reservoir rock. Recent discoveries indicate that oil production is mainly from the highly permeable, fractured, silica-rich sections. It is therefore important to recognize the diagenetic boundaries on seismic records and tomore » delineate the more brittle quartz-rich facies where the reservoir quality is expected to be better than the intermediate Opal-A or Opal-CT facies. Furthermore, these boundaries could also provide good diagenetic traps off the flanks of structures where updip unaltered impermeable rocks hinder fluid migration.« less

Paleoecology of Middle and Late Miocene Monterey Formation, Upper Newport Bay, Newport Beach, California: ABSTRACT

The Miocene Monterey Formation exposed along the cliffs of Upper Newport Bay represents a lower calcareous facies dominated by foraminifera and coccoliths, indicating warm climate and low nutrient waters, and a siliceous facies consisting of diatomaceous rocks, which records high productivity as a result of climatic deterioration and intensified upwelling. Using the distribution and preservation of foraminifera in the California continental borderland basins as a modern analog, they interpret the effect of changing environmental (climatic and oceanographic) conditions during the middle and late Miocene on the distribution and preservation of foraminifera, calcareous nannofossils, radiolarians, diatoms, and sediments. Two stratigraphic sections were sampled: (1) middle Miocene (Luisian) section 1, and (2) late Miocene (upper Mohnian) section 2. Section 1 best correlates with the middle Miocene Sphenolithus heteromorphus nannofossil zone. Benthic foraminifera indicate deposition in upper bathyal to upper middle bathyal depths. Poor preservation of calcareous tests is generally associated with homogeneous sediments, whereas better preservation is correlated with indistinctly laminated rocks. Stratigraphic section 2, which accumulated in upper bathyal depths, covers parts of the early late Miocene (upper Mohnian) Denticulopsis hustedtii-D. lauta and D. hustedtii diatom zones. Foraminifera and nannofossils are generally more abundant and better preserved in well laminated sedimentsmore » than in sediments with homogeneous to indistinct laminations. Benthic foraminifera favoring low oxygen conditions have low diversities in rocks with poorly preserved laminations, but have higher densities in well laminated intervals.« less

An image-enhancing oil technique for friable, diatomaceous rocks

An image-enhancing oil technique for friable, diatomaceous rocks

Diagenetic trends in the siliceous facies of the Monterey Shale in the Santa Maria region, California

ABSTRACTX‐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.

Porosity Reduction During Diagenesis of Monterey Formation, Santa Barbara, California: ABSTRACT

In the Monterey Formation of the Santa Barbara coastal area, porosity decreases with increased burial diagenesis from an average of 60% in diatomaceous (opal-A) rocks to an average of 10% in rocks bearing diagenetic quartz. In carbonate-free siliceous rocks, porosity was lost principally in two abrupt reductions of 10 to 30% porosity during the two silica phase transformations (opal-A to opal-CT and opal-CT to quartz). Large differences in porosity among interbedded rocks with different silica phases show that porosity losses resulted directly from the two silica phase transformations rather than from specific conditions of temperature and burial depth. Porosity reduction was thus probably due to brief loss of strength during solution-precipitation of silica in the rocks. Similar compositions and different thicknesses of sedimentary End_Page 940------------------------------ units in adjacent stratigraphically equivalent sections with different silica phases show that most porosity reduction during these transformations resulted from compaction, not from addition of silica. In calcite-bearing siliceous rocks, the pattern of porosity reduction was similar, and minor additional reduction was caused by silica filling of some foraminiferal tests. In calcite-bearing rocks, with moderate (10 to 40%) silica, only 5 to 10% of porosity was lost during each silica phase transformation, and additional gradual compaction occurred in the interval between the two transformations, presumably in response to increased burial load. Formation of disseminated dolomite, in contrast to the rarer formation of highly cemented dolomite beds and nodules, was accompanied by only moderate reduction in porosity (0 to 10% porosity). End_of_Article - Last_Page 941------------

Lithofacies and Depositional Environments of Monterey Shale, California: ABSTRACT

The Miocene Monterey Formation records the deep basinal phase of a major late Tertiary cycle of basin formation and filling associated with wrench fault tectonism. Over much of coastal California, Neogene facies show a progression from Oligocene nonmarine and neritic rocks to Miocene basinal shales and siliceous rocks and then to upper Miocene to Pliocene turbiditic and neritic clastic rocks. In many places the Monterey consists of a basal calcareous facies, a middle transitional phosphatic facies, and an upper thick siliceous facies composed of diatomaceous rocks and their diagnetic equivalents (cherts, porcelanites, etc). Hydrocarbon-producing turbidite sandstones occur in these basinal rocks in the southern San Joaquin Valley and in redeposited pelletal phosphorites in the central Coast Ranges. By analogy with modern environments of phosphate formation off South America and Africa, Monterey phosphatic shales probably represent phosphatization of shelf-slope-basin muds near the boundaries of the oxygen minimum zone. Nodular and pelletal phosphorites may also have formed near the boundaries of the oxygen minimum zone, but on sediment-starved bank tops and shelves. The widespread siliceous facies represents rapidly deposited diatom ooze that records high plankton productivity spawned by late Miocene climatic deterioration and intensified upwelling. Abundant organic matter, ecologic inferences from faunal data, and sedimentary structures such as alternations of massive and laminated cycles indicate that these siliceous rocks also formed as basin, slope, and shelf deposits within or near the fluctuating boundaries of a well-developed oxygen minimum zone--a depositional environment similar to the present Gulf of California and western margin of South America. End_of_Article - Last_Page 929------------

Montery Rocks Along Santa Barbara Coast, California: ABSTRACT

Along the Santa Barbara coast, field characteristics of individual Monterey rock vary greatly, relating to two principal factors: silica phase and bulk chemical composition. In terms of silica phase, field characteristics are most affected by whether silica is dominantly biogenous (opal-A) or diagenetic (either opal-CT or quartz). Diatomaceous rocks differ from compositionally equivalent rocks bearing abundant diagenetic silica in hardness, density, cohesiveness, surface texture, and resistance to erosion. By contrast, opal-CT rocks differ from compositionally equivalent quartz rocks mainly in bulk density, and differentiation between the two groups is usually impractical in the field. Monterey rocks contain: biogenous or diagenetic silica (5-90%), detrital minerals (5-70%), carbonate rocks (0-80%), apatite (0-30%), and (carbonaceous) organic matter (1-25%). Field characteristics are affected mainly by the silica/detrital ratio. As this ratio decreases among diatomaceous rocks, bulk density and color saturation (darkness) tend to increase. As the ratio decreases End_Page 444------------------------------ among rocks with abundant diagenetic silica, effects are more pronounced: bulk density systematically varies; cohesiveness, hardness, and brittleness all decrease. Carbonate rocks have comparatively little effect, although cohesiveness and resistance to erosion increase somewhat with the presence of dolomite. Rocks most likely to be misinterpreted are: (1) quartz porcellanites (due to matte surface, viewed as opal-CT rocks); (2) opal-CT cherts (due to vitreous surface, viewed as quartz rocks); (3) carbonate-bearing mudstones with low (10-15%) detrital contents and high (60-80%) diagenetic silica contents (due to resistance to erosion, viewed as dolomites or limestones); (4) organic shales with low (10-25%) diagenetic silica contents (due to friability of weathered rock, viewed as diatomaceous--even where silica is quartz). Overall lithologic characteristics suggest that diagenetically-produced boundaries may be more prominent, geophysically, than stratigraphic boundaries within--or formational contacts of--the Monterey Formation. Facies trends interpreted from diagenetic boundaries, however, may be highly misleading. End_of_Article - Last_Page 445------------

An Upper Miocene Diatomaceous Rock Unit on the Oregon Continental Shelf

Samples of diatomaceous sedimentary rock from the Oregon continental shelf appear to have come from a definable stratigraphic unit. This unit may be, in part, a correlative of the Pullen Formation of northern California. Diatoms and silicoflagellates from the offshore rock unit suggest a Delmontian (late Miocene) age. This occurrence of diatomaceous upper Miocene rock on the Oregon shelf adds to the growing list of such occurrences in the upper Miocene around the Pacific.

Eocene Volcanics in South-east England

IN Britain the contrast between the geology of the Tertiary Volcanic Province of northern Ireland and western Scotland and the Tertiary Sedimentary Province of south-eastern England is very marked. In the English Lower Eocene, the familiar London Clay was deposited in the south-western part of a larger marine “North Sea Basin”1, and on the eastern side of this “London Clay” deposits are known in Denmark, north-western Germany, Netherlands and Belgium. In Denmark, however, a conspicuous series of marine volcanic ashbeds, interbedded with diatomaceous rock, is seen in the Lower “London Clay” of Jutland2,3, and is known from many borings in north-western Germany4. A thin marginal ashfall is preserved subsurface in the northern Netherlands, which is also in the lower part of the Lower Eocene5.

Notes on the study of fossil diatoms

This short paper treats the subject of fossil diatoms from the geological standpoint. If we would read the history of subsurface. it is our part to learn to decipher the records in the sediments correctly. Much of this has been done, but much also remains to be done.Micropaleontology is very largely concerned with the study and classification of foraminifera, and other microscopic fossils of value in this connection are diatoms, ostracods, radiolaria, silicoflagellata, etc.Diatoms as micro-fossil are commonly liable to neglect in Geology. But they also afford testimony as to the environment in which they lived. Every species of diatoms also have a given home or environment known as a habitat, which may be marshy land, rivers and lakes, or seas and oceans.Owing to the very great number of known species and the difficulty of obtaining sufficiently reliable data on their stratigraphic distribution, the specific determination of fossil diatoms and their stratigraphic use is a particularly difficult task which has been carried out successfully only by a limited number of specialists.Many deposits such as mudstons, fine-grained shales or tuffs have fossil diatoms, or certain combinations, of their species, peculiar to themself, these can be used in correlating the strata of a given age from place to place or even land to land. wide distribution of Pelagic diatoms make them valuable index-fossils for long range correlation such as the diatomaceous bed in the Tertiary along the coast of Japan Sea. This paper has given some examples of local correlation by means of a distribution between a marine and a non-marine diatom flora occuring in lithologically similar diatomaceous rocks encountered in well cores. Diatomaceous rocks occur in the natural gas fields of Japan and diatoms make use sufficiently in these areas for stratigraphic correlations.In the fall of 1940 a. large number of test wells where put down in many areas of Japan, for the purpose of determining the possibility of developing the natural gas supply instead of oil fuel. Some samples of these wells were collected and send to me for examination. A great many of these samples contained either foraminifera, diatoms, or other micro-fossils such as silicoflagellata, radiolvia, sponge specules, pollens, etc. Like all other fossils, some species are found ranging through several deposits, whereas others are more narrowly limited. Deposits can sometimes be recognized and distinguished by a characteristic species, or more often by a particular assemblage of species, different from the species in other associated strata.The geological reports of fossil diatoms are commonly divided into two parts. The first part deals with the distribution of diatoms in the rocks, with a discussion of their ecologic and geologic significance; the second Part gives a systematic description of the diatoms. Now, it is worthy of our notice that electron-microscope is widely used on the frustules of diatoms. Such systematic descriptions on the ground of electron-microscope make in future rapid progress for the study of both living and fossil diatoms.In conclusion, I have a word to say in regard to the future. Biologist, micropaIeontologist and geologist should work in harmony to attain the accurate and complete knowledge of many events in the past.

Occurrence and Origin of Chert in the Monterey Formation: ABSTRACT

Chert is less common than the other siliceous rocks in the Monterey but is locally prominent, especially in lower parts of the formation. Evidence is presented indicating that most of the siliceous rocks were formed through an alteration of originally diatomaceous rocks, the chert forming from beds and areas of relatively pure diatomaceous deposits. Most of this alteration by solution and recementation was after burial of the deposits. Surface rocks commonly show recementation of the fractured cherts. More open fractures might persist, however, if accumulation of oil were nearly coincident with some of the structural deformation and fracturing. End_of_Article - Last_Page 2239------------