Basin Sedimentary Rocks Research Articles

Relationships between porosity and effective stress in shales

In order to predict overpressures by basin modelling, it is essential to understand the process of mechanical compaction and its effect on the porosities and permeabilities of sedimentary rocks. Two fundamental issues for modelling overpressure history are the functional relationships between porosity and effective stress, and between permeability and porosity. This article is concerned with the first of these relationships in shales, since they comprise the majority of rocks in sedimentary basins and their low permeabilities restrict the rate at which excess fluid pressure can be dissipated. My key point is that porosity reduction during mechanical compaction of a sediment depends on the mean effective stress, not the vertical effective stress. This must be recognized if we wish to understand the origin of overpressures observed in sedimentary basins.

Petrogenesis of juvenile-type Birimian (Paleoproterozoic) granitoids in Central Côte-d'Ivoire, West Africa: geochemistry and geochronology

The Birimian (Paleoproterozoic) granitoids of Central Côte-d'Ivoire are associated either with greenstone belts or with basin sedimentary rocks. Field data indicate two stages of granitoid emplacement which, according to singlezircon and monazite 207 Pb 206 Pb age determinations, occurred within a short period of time. The first generation of granitoids (2123-2108 Ma) intruded and metamorphosed the greenstone belts, whereas the second generation (2108-2097 Ma) intruded and metamorphosed the sedimentary pile and subordinate andesitic volcanic rocks of the Bandama basin unconformably overlying the greenstone belts. Geochemical and isotopic data show that the various granitoids belong to four magmatic groups: (1) sodic calc-alkaline (NaCG); (2) potassic calc-alkaline (KCG); (3) peraluminous (AlG); and (4) alkaline (alkG). All the granitoids show juvenile features, with mantle-like low initial Sr isotopic compositions (0.700-0.702) and positive epsilon Nd values (1.5-2.7). These data indicate that (a) the NaCG may have formed by partial melting of a garnet-amphibolite crust, (b) the KCG and alkG derived from partial melting of crustal, mafic to intermediate metaigneous rocks and (c) the AlG (epsilon Nd = 1.5) represent infracrustal melting of young sialic igneous materials during a regional crustal shortening, in a transcurrent tectonic context. SmNd model ages (2.25-2.4 Ga) preclude the existence of an Archean crust reworked in the Birimian magmas. An underplating model is discussed.

The Oquirrh Basin Revisited

The upper Paleozoic succession in the Oquirrh basin is unusually thick, up to 9300 m, and consists mainly of a Pennsylvanian-middle Permian sedimentary package located within the Cordilleran miogeocline of northwestern Utah. Previous workers have suggested a tectonic origin for the Oquirrh basin that is incompatible with the basin location in both time and space. There is no evidence for Pennsylvanian and Lower Permian tectonism in the middle of the miogeocline. Thermal evidence from the Mississippian Mission Canyon shale does not support the implied deep burial of the crustal sag models of basin formation. Stratigraphic and facies evidence indicates a growth fault origin for the basin. Regional isopach maps and facies maps are powerful tools in interpreting depositional environments and in reconstructing fold-and-thrust belts. However, the location of measured sections relative to the location of the growth fault needs special consideration for these tools to be effective in reconstructing a thrusted growth fault basin. The Charleston-Nebo thrust may have essentially reversed the movement on a growth fault. Thick Oquirrh basin sedimentary rocks may not be required to balance structural sections across this thrust fault. A thin-skinned, extensional growth fault origin for the Oquirrh basin implies that the Cordilleran miogeocline did not participate in the Pennsylvanian north-vergent uplifts of the Ancestral Rocky Mountains.

Artificial maturation of alginite and organic groundmass separated from torbanites

The two principal organic constituents— Botryococcus-related alginite and organic groundmass—were isolated by density separation from two torbanite samples (from the Stellarton Formation, Nova Scotia, Canada and the King Cannel, Utah, U.S.A.). The groundmass consisted of degraded algal, bacterial and terrestrial plant debris. Aliquots of alginite and groundmass were separately heated in gold tubes for 24 h with 70 MPa confining pressure, at fixed temperatures ranging between 250 and 375°C. The 250, 300 and 325°C experiments run on the alginite produced very low yields of CHCl 3-extractable organic matter (EOM), indicating that very little of the generation potential had been tapped. The alginite reached the onset of generation at 350°C and peaked at 375°C. The groundmass exhibited a distinctly different response to heating. Its 300, 325 and 350°C experiments showed a progressive increase in EOM yield with increasing temperature, producing more EOM than the corresponding alginite runs, in spite of the lower initial generation potential of the groundmass. However, EOM yields were lower at 375°C, indicating that its peak generation had occurred at 350°C. After heating, the CHCl 3-extracted residues were analyzed by Rock-Eval and flash pyrolysis-GC-MS to determine the remaining petroleum potential and monitor the alterations in the macromolecular structure. In nature, petroleum generated from a torbanite would be a mixture of the liquids generated by each of its components, in a blend that would change as thermal alteration progressed, as the various constituents each reached their peak of generation. Such a multi-component model of torbanite composition can serve to improve predictions of oil generation from torbanites and related source rocks in sedimentary basins.

Permo-Triassic sedimentary and volcanic rocks in basins to the north and west of Scotland

Abstract Red-bed sequences of known or inferred Permian and Triassic age are preserved in numerous offshore basins to the north and west of Scotland. These developed following the Late Carboniferous Variscan Orogeny, as compressive uplift was superseded by a tensional stress regime causing the fragmentation of the Variscan foldbelt and its northern foreland. Basin formation persisted through the Permian and Triassic periods, and the sediment infills reflect syn- and post-depositional fault activity. Syndepositional basins take the form of deep, fault-bounded half-grabens whose development was controlled by the extensional reactivation of earlier (Precambrian/Caledonian) thrusts and transfer faults. These basins accumulated wedge-shaped packages of sediment dipping into major basin-bounding faults. In contrast, post-depositional basins are characterized by a parallel-bedded infill which appears to reflect local preservation as a result of later faulting. These basins may be Late or post-Triassic in age. The Permo-Triassic rocks were deposited during a period of major regression, with fluviatile, lacustrine and aeolian environments predominating. Principal lithologies include sandstones, that are partly conglomeratic, intercalated with mudstones and claystones. The localized development of carbonates and evaporites indicates some marine influence. Palaeontological data are commonly sparse throughout the succession and generally cannot readily differentiate the two systems. There is limited evidence for both Early Permian and Late Triassic volcanism. The Early Permian volcanism was ‘within-plate’ and ‘continental’ in character, and was probably related to the first phase of post-Variscan continental disintegration. The Late Triassic volcanism indicates a subsequent episode of synrift igneous activity west of Britain and Ireland.

Temporal changes in Nd isotopic composition of sedimentary rocks in the Sevier and Taconic foreland basins: Increasing influence of juvenile sources

Initial 143 Nd/ 144 Nd ratios of clastic sedimentary rocks in the Ordovician Taconic and Sevier foreland basins increase with decreasing stratigraphic age. This shift in isotopic composition reflects an increased influx of juvenile sediment to both basins. For the Taconic basin, geologic and isotopic data are consistent with the juvenile sediment being derivedfromupliftedGrenvillecrust,upthrustaspartoftheTaconicallochthons,sediment derivedfromtheBronsonHillarc,oracombinationofboth.FortheSevierforelandbasin, there are few geologic constraints on source regions, but the similar shift in isotopic ratios with time suggests a similar increase in the influence of juvenile sources such as exotic terranes. Sevier basin sedimentary rocks have higher «Nd values than those within the Taconic basin, thus juvenile source areas were of greater importance to Sevier basin sediments.

Timing of mineralization and deformation, Peak Au mine, Cobar, New South Wales∗

The 40Ar/39Ar dating of alteration muscovite from the Peak Au mine in the Early Devonian Cobar Basin, New South Wales, has distinguished two major episodes of mineralization. Veined (Pb‐Zn‐Cu‐Au) mineralization was broadly synchronous with cleavage formation during the post‐inversion, shortening deformation of the basin sedimentary rocks, and replacive Ag‐Pb‐Zn mineralization significantly postdates the latter event. Veined base metals (Pb‐Zn‐Cu) and Au associated with silicification were coeval with three stages of cleavage formation (D1, D2 and D3) after basin inversion. The Cu‐Au phase of mineralization at the Peak Au mine which was broadly contemporaneous with the culmination of the cleavage‐forming events (D3) and with the local development of high‐strain zones occurred at 401.5 ± 1.0 Ma (40Ar/39Ar on muscovite). This date is essentially coeval with known fossil constraints on the age of basin formation, and indicates that basin inversion and deformation rapidly followed sedimentation. In contrast, replacive Ag‐Pb‐Zn mineralization occurred at 384.0 ± 1.4 Ma (40Ar/39Ar on muscovite) during an extended period of relaxation characterized by normal faulting (D4) which followed the shortening deformation. This mineralization was associated with desilicification and chlorite‐muscovite replacement assemblages which cross‐cut the cleavages, and which may have been broadly contemporaneous with the deposition of part of the Mulga Downs Group which unconformably overlies the Cobar Supergroup. Rhyolite exposed in the core of the Peak Au mine largely contains inherited zircons that range in age from ∼430–1500 Ma. A few euhedral zircons have an age of ∼430 Ma and this is interpreted as a maximum date for the rock. Zircons from a syn‐D3 chlorite‐muscovite replacement zone within the deposit have 206Pb/238U ages of ∼410–650 Ma and are apparently inherited.

Paleomagnetism of the Sassamansville diabase, Newark Basin, southeastern Pennsylvania: Support for Middle Jurassic high-latitude paleopoles for North America

A paleomagnetic study of the Sassamansville diabase, which intrudes the Passaic Formation red beds of the Newark Basin in southeastern Pennsylvania, was conducted to constrain the age of the diabase9s magnetization. Nine sites were collected from the diabase around the Sassamansville syncline. A tenth site was collected at an associated dike 12 km to the north. Eight to ten individually oriented cores were drilled at each site. Alternating field (af) and thermal demagnetization isolated a stable, internally consistent characteristic remanence at eight sites. Using the bedding tilt of the nearest baked or unbaked Passaic Formation sedimentary rocks for a fold test, the site means showed best clustering at 65% unfolding. This magnetization is a statistically significant (at 93% confidence) synfolding magnetization. The diabase9s magnetization fails a baked contact test conducted at one of the sites, indicating that the magnetization is secondary and not a primary thermal remanent magnetization (TRM). Isothermal remanent magnetization (IRM) and partial anhysteretic remanent magnetization (pARM) acquisition experiments and of demagnetization results indicate that the magnetization is carried by high-coercivity magnetite. Anisotropy of magnetic susceptibility (AMS) and anhysteretic remanence (AAR) results indicate that this magnetite has a different fabric from the northeast-southwest horizontally lineated fabric carried by low-coercivity magnetite grains that do not carry the remanence. The lineated fabric may have been caused by flow of the diabase from the north-east to the southwest during emplacement. When the rock magnetic results are considered in light of previous petrographic observations (Davidson and Wyllie, 1968) of these rocks, the rock magnetics suggest that the magnetization is carried by secondary magnetite, which has grown as rims on primary magnetite. The secondary magnetite may have formed during a hydrothermal event proposed by Sutter (1988) based on geochronologic evidence and dated at 175 Ma. If the diabase magnetization is considered to be coeval to a widespread remagnetization (B component) of the Newark Basin sedimentary rocks (Witte and Kent, 1991), the diabase magnetization indicates 15° of counter-clockwise rotation for the Sassamansville syncline. This is consistent with either left-lateral motion along the border fault or along the intrabasinal Chalfont Fault. The presence of counterclockwise block rotations along the border fault could weaken previously reported evidence that the Newark Basin (B component) remagnetization occurred after the Newark Basin strata had acquired most of their north-westerly tilt (Witte and Kent, 1991). Correc-tion for the tilt would move the Newark (B component) remagnetization paleopole to lower paleolatitudes but necessitate significant post-175 Matectonic activity in the Newark Basin. This would contradict the long sedimentary record of rifting and border fault activity for the preceding 50 m.y. A partial tilt correction may be the best geologic and paleomagnetic interpretation because it would minimize the amount of post-175 Ma tectonic activity required while bringing the Newark (B component) remagnetization paleopole into paleolatitudinal agreement with a paleopole derived from the rotation of European, South American, and African data into North American coordinates (Van der Voo, 1992).

Inversion of the Proterozoic Wernecke basin during tectonic development of the Racklan Orogen, northwest Canada

New deep seismic reflection data coupled with regional stratigraphic correlations, drill-hole information, and potential field data are interpreted to provide images of Middle Proterozoic Wernecke Supergroup (meta-)sedimentary layers that were uplifted during tectonic development of the ca. 0.9–1.3 Ga Racklan Orogen in Canada's western Northwest Territories. The reflection data are located at the eastern front of the Mackenzie Mountains portion of the Canadian Cordillera and on the western flank of the Fort Simpson structural trend that is a prominent Proterozoic structure in the subsurface throughout the region. Along three parallel profiles, layers that are correlated with thick Wernecke Supergroup sedimentary rocks produce prominent reflections between about 3.0 and 9.0 s (about 7.5 and 23 km) that were arched prior to deposition of younger Proterozoic (probably Mackenzie Mountains Supergroup) and Phanerozoic sedimentary rocks. The strata are considered to be Wernecke basin sedimentary rocks that were uplifted during deformation associated with the development of the Racklan Orogen.

Eoalpine to Neoalpine magmatic and metamorphic processes in the northwestern Vardar Zone, the easternmost Periadriatic Zone and the southwestern Pannonian Basin

The occurrence of Eoalpine-Neoalpine, genetically different, granitoids and penecontemporaneous volcanics are characteristic for the Vardar Zone. These rocks are mostly associated with Late Cretaceous-Paleogene basinal sedimentary rocks underlain by ophiolites. The Late Cretaceous-Paleogene rocks were progressively metamorphosed under mediumpressure conditions and intruded by synkinematic Mesoalpine granitoids. The high-pressure blueschist metamorphism was related to a pre-Maastrichtian Eoalpine metamorphic event. Eoalpine and Mesoalpine magmatism and the related metamorphism of the Vardar Zone may have taken place along a magmatic arc, i.e., the subduction zone. The easternmost parts of the Periadriatic Zone are characterized only by Oligocene granitoids accompanied by penecontemporaneous volcanic rocks. Here, younger Alpine metamorphic phases only overprint and mylonitize older pre-Alpine formations. Most of the Mesoalpine granite plutonism both of the Vardar Zone and the easternmost area of the Periadriatic Zone was related to subsequent extension. The southwestern and southern parts of the Pannonian Basin contain the products of four pulses of Mesoalpine and Neoalpine synsedimentary volcanic activity (Egerian-Eggenburgian, Ottnangian-Carpathian, Badenian and post-Badenian) represented by predominant basalt-andesites, trachyandesites and trachydacites. This post-subduction volcanism was closely connected with the cyclic marine ingressions in the Pannonian Basin.

Oil Source Rocks in Sedimentary Basins of the CIS

Oil Source Rocks in Sedimentary Basins of the CIS

A mathematical model for compaction in sedimentary basins

SUMMARY A mathematical model for the non-equilibrium compaction of clay rocks in sedimentary basins is formulated. The model generalizes those of earlier authors. The simplest assumptions are made concerning the rheology, and diagenesis is neglected. In this case, we show that the model reduces to a generalized consolidation equation, which for the classical Darcy flow is a non-linear diffusion equation for the porosity, with a free boundary. The model is non-dimensionalized, and it is shown that solutions depend on one significant dimensionless parameter A, which is the ratio of the Darcy flow rate and the sedimentation rate. The model is solved numerically, and asymptotic descriptions of the solutions are given for the cases of large and small A.

Late Paleozoic to Triassic evolution of the Gondwana margin: Evidence from Chilean Frontal Cordilleran batholiths (28°S to 31°S)

Upper Paleozoic to Triassic Chilean granitoids in the Andean Frontal Cordillera between 28°S and 31°S record crustal and mantle conditions at the Gondwana margin during the final assembly and initial breakup of the Pangea supercontinent. This period overlaps the end of Paleozoic terrane accretion and precedes Andean subduction. Integration of new trace-element and isotopic data with other information on the granitoids and the regional geology leads to a tectonic model that has implications for other parts of the Gondwana margin. In the model, the Carboniferous to Early Permian is a period of oblique convergence. Associated Elqui complex granitoids are diverse. Those in the Guanta and Montosa units are predominantly related to subduction processes, whereas those in the Cochiguas and El Volcan units are dominated by melting of the subduction complex and older crust. Progressive oblique collision of the last pre-Pangea terrane (Equis) along the margin resulted in crustal thickening associated with shortening deformation of foreland basinal sedimentary rocks and uplift of the Elqui complex. Subsequent gravitational collapse of the inactive slab and lithospheric delamination resulted in the production of large amounts of basalt, which intruded and melted the crust, producing the post-collisional Ingaguas complex. The Los Carricitos granitoids formed in thickened crust, whereas the Chollay, El Colorado, and El Leon units formed in thinner crust. The Ingaguas complex is part of the Choiyoi granite-rhyolite province, whose formation, similar to that of other Gondwana silicic provinces, was probably accentuated by anomalously hot upper mantle associated with the Pangea supercontinent.

Opposite thrust systems in northern Victoria Land, Antarctica: Imprints of Gondwana's Paleozoic accretion

Two major thrust systems with contrasting senses of displacement transect the Wilson terrane crust of northern Victoria Land, Antarctica. Along both mylonitic shear zones the central high-grade metamorphic basement is detached and thrust divergently toward the west and east over synorogenic, lower grade fore-arc and back-arc basin sedimentary rocks, respectively. Deformation was preceded by pervasive high-temperature-low-pressure metamorphism. Granites intruded the basement prekinematically and postkinematically. The structures are interpreted as results of early Paleozoic subduction of the paleo-Pacific oceanic crust under the Antarctic craton.

Carbon dioxide in clastic rocks and silicate hydrolysis

The amount of carbon dioxide in clastic rocks in sedimentary basins increases with depth. Organic matter, dissolved organic species, and dissolution of carbonate minerals have been suggested as sources of CO{sub 2}, which increases in abundance with depth and temperature. Isotopic compositions of aqueous HCO{sub 3}{sup {minus}}, gaseous CO{sub 2}, and calcite from conventional hydrocarbon wells and steam-assisted recovery of heavy oil suggest that calcite, or other carbonate minerals, are the source of CO{sub 2}, particularly at higher temperatures. Detailed examination of the stability of diagenetic minerals such as kaolinite, smectite, and analcime, among others, relative to the coexisting waters, indicates that silicate hydrolysis is the driving force for dissolution of carbonate minerals and the ultimate source of abundant CO{sub 2} in clastic rocks in diagenetic environments at temperatures over approximately 100 C.

Mantle plumes and flood-basalt stratigraphy in the Paraná, South America

Research Article| December 01, 1990 Mantle plumes and flood-basalt stratigraphy in the Paraná, South America David W. Peate; David W. Peate 1Department of Earth Sciences, Open University, Milton Keynes, MK7 6AA, England Search for other works by this author on: GSW Google Scholar Chris J. Hawkesworth; Chris J. Hawkesworth 1Department of Earth Sciences, Open University, Milton Keynes, MK7 6AA, England Search for other works by this author on: GSW Google Scholar Marta S.M. Mantovani; Marta S.M. Mantovani 2Departamento de Geofísico, Instituto Astronômico e Geofísico, Universidade de São Paulo, Caixa Postal 30627, 01051 São Paulo SP, Brazil Search for other works by this author on: GSW Google Scholar Wladimir Shukowsky Wladimir Shukowsky 2Departamento de Geofísico, Instituto Astronômico e Geofísico, Universidade de São Paulo, Caixa Postal 30627, 01051 São Paulo SP, Brazil Search for other works by this author on: GSW Google Scholar Geology (1990) 18 (12): 1223–1226. https://doi.org/10.1130/0091-7613(1990)018<1223:MPAFBS>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David W. Peate, Chris J. Hawkesworth, Marta S.M. Mantovani, Wladimir Shukowsky; Mantle plumes and flood-basalt stratigraphy in the Paraná, South America. Geology 1990;; 18 (12): 1223–1226. doi: https://doi.org/10.1130/0091-7613(1990)018<1223:MPAFBS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Geochemical studies of the Paraná continental flood basalts in Brazil have led to the recognition of distinct magma types, which have been used to infer the internal stratigraphy of the lava pile. The overstepping of stratigraphic units toward the north and a similar compositional change in sills within the underlying Paraná basin sedimentary rocks imply that the site of magnatism migrated ∼750 km toward the north during this volcanic event. This has important implications for recent plume-related models that have argued whether fithospheric rifting must accompany the presence of a mantle plume in order to generate continental flood-basalt magmatism. It is difficult to ascribe the magnitude and direction of the shift in the locus of the Paraná magmatism to movement of the Brazilian lithosphere relative to the underlying Tristan mantle plume. Instead, it is suggested that the observed lava distribution and internal structure of the Paraná-Etendeka flood-basalt province were imposed by the intracacies of the rifting process during the initial opening of the South Atlantic Ocean. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Are Hydrocarbon Source Rocks Hidden Beneath Volcanic Flows in the Southern Washington Cascades?: ABSTRACT

Magnetotelluric surveys in the southern Washington Cascades have outlined a thick wedge of conductive rocks interpreted to represent a previously unmapped complex of marine sedimentary rocks. These postulated sedimentary rocks occur at depths of 1-10 km and with a thickness of up to 10 km. Shallow depths of 1-2 km to the top of the conductive section occur along anticlines that are evident in deep reflection data and are known from surface geological mapping. These anticlines are cored with Eocene marine formations like the McIntosh, Carbonado, and Raging River formations, and the authors infer that these formations and older, unmapped marine units make up the bulk of the conductive package. The marine rocks are also interpreted to have been deposited in the fore-arc environment of a subduction zone that was forced to reposition westward in the middle Eocene by accretion of a large seamount complex. Other lithologies that may be part of the conductive package are (1) highly altered volcanic flows with geothermal fluids; (2) continental pull-apart basin sedimentary rocks; or (3) pre-Tertiary sedimentary rocks like those in the Rimrock Lake inlier. Factors such as possible organic content, maturity, and reservoir quality have been reviewed for the authors proposed model.more » This review shows that the hypothesized marine complex should be mature over most of its extent and that organic carbon content may be high due to anoxic conditions in the assumed, restricted fore-arc environment. Structural traps may occur in several anticlines in the study area.« less

Seismic Velocity Variations in the Northern Amadeus Basin, Central Australia, from an Expanding Spread Reflection Profile

An expanding reflection spread with a maximum shot-receiver offset of 25 km was recorded in the northern part of the Amadeus Basin, Central Australia, to obtain seismic velocity estimates throughout the crust. Arrival times of reflections indicate significant lateral variations in P-wave velocities superimposed on the small northerly dip of the sedimentary sequence. Nevertheless, a useful velocity profile that contains two pronounced low velocity zones has been derived for the basin sedimentary rocks to depths of 8.6 km. Below depths of 3 km, these interval velocities are a considerable improvement on those obtainable by standard velocity analysis techniques on the coincident near-vertical incidence reflection data; the results thus enable the determination of more reliable estimates of the thicknesses of the deeper sedimentary formations. The refracted arrivals from the expanding spread yield well-constrained P-wave velocities for shallow depths, but below about 1.5 km depth, the refraction interpretation is problematic owing to the absence of a refracted arrival through the high velocity lid in the interval velocities at depths between 1.8 and 2.7 km. The determination of useful velocity estimates for basement rocks is hampered by the presence of peg-leg multiples produced within the overlying sedimentary formations.

Cementation and Compaction History of Synorogenic Foreland Basin Sedimentary Rocks from Huaco, Argentina

The Sierra de Huaco exposes the western flank of the Bermejo foreland basin of central western Argentina. The exposed 5,400-m section is well dated (14-2.3 Ma) and consists of synorogenic continental strata. Petrographic data combined with decompaction techniques provide first-order estimates of the absolute age of cementation of the sandstones. This information can be used to interpret petroleum migration paths and reservoir potential. Three diagenetic zones have been recognized, each characterized by a dominant cementing material within a distinct framework texture. These textures reflect varying degrees of compaction and framework-grain replacement prior to cementation. The intergrain volume in the sandstones increases upward in a nonlinear, nonexponential fashion and is interpreted to reflect the porosity at the time of cementation. The lowest 550 m of strata exhibits abundant compaction features, low intergrain volume (~10%), and is cemented by analcime. The overlying 900 m exhibits little evidence of compaction, relatively high intergrain volume (up to 33%), and is cemented by gypsum. The remaining 4,000 m exhibits abundant evidence of framework-grain replacement by calcite, has very high intergrain volume (up o 40%), and is cemented by calcite. The diagenetic histories of the three zones were genetically linked to thrusting in the adjacent Precordillera, changes in depositional environment, and subsurface fluid flow. The depth at which cementation occurred in each zone is constrained by comparison of observed intergrain volume to predicted volumes (for uncemented sands) at any given burial depth. First-order estimates of the absolute age of cementation in each zone were made possible by comparing these relationships with a curve of accumulation history of the decompacted strata. The lowest zone compacted with little interstitial cement for as long as 11 m.y. after deposition. The middle zone was cemented within 3 m.y. after deposition. The upper zone experienced framework-grain replacement by calcite at very shallow depths w thin 2 m.y. and experienced little compaction.

Relationship Between Relative Sea Level Fluctuations and Petrologic Variation in Forearc Basin Sedimentary Rocks of Upper Cretaceous Rosario Formation at San Carlos, Baja California del Norte, Mexico: ABSTRACT

Petrographic study of a well-exposed submarine canyon fill sequence indicates that petrologic variation corresponds to changes in relative sea level. These changes are recorded as textural variation in the canyon fill: a basal coarse-grained unit capped by a middle fine-grained unit records a rise in sea level, and an upper coarse-grained unit records a temporary fall in sea level. These sea level fluctuations do not correlate with published eustatic sea level curves, although the temporary fall may be recorded on a regional (San Diego to San Carlos, 400 km) scale. Sandstone samples from the lower and upper coarse-grained units contain higher concentrations of volcanic rock fragments and plagioclase than the middle fine-grained unit, whereas the middle fine-grained unit contains higher concentrations of quartz and K-feldspar. Fluctuations in relative sea level may have been controlled by tectonic or magmatic events: (1) a tectonically controlled rise in relative sea level could have resulted in longer residence tie of the sediment in a nearshore environment, causing increased mechanical and chemical weathering of the least-resistant grains (i.e., plagioclase and volcanic fragments) relative to quartz and K-feldspar; or (2) a temporary lull in volcanism could have resulted in increased dissection of the arc and decreasedmore » sediment input due to lowered base level within the arc, resulting in a marine transgression and an increase in the concentration of quartz and K-feldspar in the sediment. The lack of significant alteration of the feldspar grains in the fine-grained unit relative to the coarse-grained units may favor the second hypothesis.« less