Active Margin Environment Research Articles

Petrostructural and Geochemical Characteristics of the Metamagmatites in the External Zone of the Dahomeyides Belt: Case of the Kantè Serpentinites (Northern Togo)

Thanks to detailed field investigations, microstructural and geochemical analysis and relationship with enclosing rocks, microfabrics, magmatic typology and metamorphic evolution of the Kantè sepentinites have been specified for the first time. The Kantè serpentinites in northern Togo constitute a mega-lens of ultrabasic rocks tectonically intercalated in the sericite chlorite schists of the Atacora structural unit. The brecciated, schitotose or massive rock facies are strongly marked by an S1 schistocity plane superimposed by a flat C shear plane linked to a west vergence thrusting movement. The parageneses that compose the metamagmatites are essentially serpentinous, containing plagioclase, opaque minerals (magnetite, chromite, spinel) and pyroxene porphyroblasts. These microfabrics represent relics of a probable gabbroic protolith. In fact, the geochemical characteristics of the Kantè serpentinites suggest that their magmatic typology is that of komatiites or tholeiitic basalts with oceanic arc affinities. They would have been emplaced in an active margin environment. The retromorphic evolution of the protolith corresponds to the phase of involvement in a major tangential contact during the panafrican tectogenesis.

Neoproterozoic amalgamation and Phanerozoic reactivation of Central/Western Hoggar (Southern Algeria, Tuareg Shield) lithosphere imaging using Magnetotelluric data

Hoggar, as part of the Tuareg shield, is a key area in the assembly of Western Gondwana during the Neoproterozoic. To better understand the tectonic process, 61 broadband magnetotelluric (MT) stations were acquired in Central/Western Hoggar to image the underlying lithosphere. We performed 2-D and 3-D inversions of the impedance tensor and the vertical magnetic field transfer function using data from three profiles. The resistive crust is interrupted by several conductors related to the known shear zones of the area. Some conductors can be associated with Cretaceous-Cenozoic fluid flow combined with mineralization (graphite, magnetite or more locally gold). MT models suggest three distinct blocks. (1) An oriental block composed of Tefedest, Laouni, Aouilène and Silet juvenile terrane characterized by a moderately conductive lithospheric mantle (LM). (2) A central block formed by the In Tedëini and Tin Zaouatene terranes, previously considered as Neoproterozoic juvenile, exhibit resistive characteristics typical of an old continental LM. Finally, (3) an occidental block corresponding to the In Ouzzal terrane with a highly conductive LM. The oriental and occidental blocks were strongly remobilized during the Neoproterozoic, and perhaps even locally during the Cretaceous and Cenozoic. The main shear zones of the study area are (1) the Iskel shear zone, a major suture zone separating the oriental and central blocks. The MT model suggests that the geometry of this lithospheric shear zone is characterized by an eastward dipping angle, according to the central block a passive margin character where Silet arc was accreted. Consequently, east from Silet a collision with the oriental block may have been in an active margin environment. (2) The Adrar fault and the West Ouzzalian shear zone correspond to terrane boundaries of the In Ouzzal. They are associated with deep-reaching high conductivity, consistent with large scale suture zones. The resistivity models provide new geometric constraints for the terranes and highlight new Pan-African and Phanerozoic tectonic features.

Slope failure repetition in active margin environments: Constraints from submarine landslides in the Hellenic fore arc, eastern Mediterranean

It has been shown that submarine landslides can occur less frequently at subduction zone fore arcs despite the general expectation of extensive slope failures from high neotectonic activity in active margin settings. The Hellenic subduction zone, Greece, represents an example where modern evidence for slope failure is scarce. Taking the deeper parts of the fore‐arc basin into account, however, a sequence of massive landslide deposits is found at recurrence intervals of approximately 250 ± 70 ka. Given high seismicity in the fore‐arc area, this rate of slope failure appears to be low. In order to improve our understanding on the relationship between low landslide recurrence rates and the frequency and settings of the required trigger mechanism, we here assess the mechanical behavior of the slope sediment cover during instability scenarios. Seismic profiles and geotechnical measurements from cores of midsize landslides found on the northeastern Cretan midslope are used to back‐analyze slope destabilization in one‐dimensional, infinite slope models for static conditions as well as for the case of seismic loading. Results reveal that today only critically steepened parts of the Cretan slope can fail from high loading stresses of peak ground acceleration (PGA) of 37%g, maybe up to ≥64%g. We further deduce that long‐term tectonic processes are important preconditioning factors controlling an occasional development of critically inclined slope parts. Therefore, the impact of seismic triggers is strongly limited in time and space: For an initially stable slope, the critical seismic intensity to trigger failure is being reduced with increasing tectonically controlled steepening of the slope through time, until an earthquake is sufficient to trigger a large landslide. Before that, the slope rather gets more resistant, because smaller PGA may rather result in dynamic compaction (seismic strengthening). Our findings imply that low frequencies of landslides in the Hellenic fore arc are not in conflict with high seismicity in this region, because seismic loading is only sufficient to trigger major collapses of a generally shear‐resistant “cohesive” slope (increasing with time due to seismic strengthening) if long‐term tectonic movement provides a critical steepening. This may explain the relatively scarce occurrence of large submarine landslides in this and similar tectonically active environments.

Geological context of the Boumnyebel talcschists (Cameroun): Inferences on the Pan-African Belt of Central Africa

The talcschists of the Boumnyebel area (southern Cameroon) form ≤ 30 m thick discontinuous layers within a Pan-African nappe unit (Yaoundé group), which includes, at the base, muscovite + biotite ± garnet micaschists associated with amphibolites and pyroxenites, and, at the top, muscovite + biotite + garnet + kyanite micaschists locally associated with marble and amphibolites. The metamorphic peak (∼650 °C/9.5 kbar; ca. 620 Ma) postdates nappe emplacement. Isograds are in normal position, micaschists passing downwards to migmatites in the northwestern part of the area studied. The rock types in the lower part of this nappe suggest active margin environments with detrital input from a nearby continental crust (arc or back-arc context).

Deciphering Caledonian events: Timing and geochemistry of the Caledonian magmatic arc in the Kyrgyz Tien Shan

In the Kyrgyz Tien Shan (also known as Tian Shan in literature) the Caledonian (Cambro-Silurian) intrusions comprise an extensive magmatic arc stretching from east to west for more than 1000 km. The characteristic feature of the arc is its relatively homogeneous composition of rock types over the whole structure. The Kichy-Naryn and Djetim intrusions presented in this study are slightly elongated in an east–west direction and occupy an area of ca. 100 km 2. The main rock types are diorite, granodiorite and granite. Geological and geochemical features of the Kichy-Naryn and Djetim intrusions demonstrate characteristics of I-type granite series. Rocks of the two intrusions define a continuous high-K calc-alkaline series. Diorite and granite of the Kichy-Naryn intrusion yielded early Silurian crystallisation ages of 436 ± 2 Ma (U–Pb, zircon). Diorite from pebbles in the conglomerate sampled close to the contact of the Kichy-Naryn intrusion yielded a significantly older early Ordovician crystallisation age of 466 ± 10 Ma. The obtained ages of 466 and 436 Ma match ages of two major regional magmatic pulses at ca. 435–440 and 460–470 Ma which took place during continuous subduction from the Cambrian to the Silurian. The amount of granites in the Northern Tien Shan, their prolonged history of formation and pronounced I-type geochemical characteristics suggest their formation in an Andean-type active margin environment.

Migração de orógenos e superposição de orogêneses: um esboço da colagem brasiliana no Sul do Cráton do São Francisco, SE - Brasil

O Orógeno Tocantins Meridional representa a pilha colisional de nappes entre três ambientes tectônicos principais (de WSW para ENE): domínio de arco magmático desenvolvido na margem continental ativa da Placa Paranapanema (Nappe Socorro-Guaxupé), domínio continental subductado (Terreno Andrelândia) e domínios com afinidades de margem passiva e/ou relacionados à Placa Sanfranciscana (Sistema de Nappes Carrancas e Nappe Lima Duarte). O magmatismo relacionado ao período de subducção remonta a 670 Ma, com auge metamórfico (geoterma relaxada na margem ativa e perturbada no terreno subductado) há 625 Ma. A atividade de arco na margem ativa foi contemporânea à sedimentação tipo-flysch no Terreno Andrelândia. Os processos metamórficos e deformacionais da etapa orogênica controlada por colisão frontal migraram para ENE de ca. 620 a 580 Ma. A duração dos processos foi"instantânea" na nappe interna ocidental (ca. 7 Ma, com plutonismo pós-orogênico há 612 Ma) e perdurou por ca. de 20 Ma na nappe oriental de alta pressão. O terreno subductado registrou rápida velocidade de exumação (ca. 2,2 mm/ano). Atividades magmáticas superimpostas, controladas por regimes extensionais (Cinturão Itu) acompanharam a exumação do orógeno e precederam a instalação de bacias sucessoras continental-marinhas há 570 Ma. O Orógeno Araçuaí, relacionado à convergência entre a margem passiva oriental Sanfranciscana e o Terreno Juiz de Fora (microplaca em ambiente de margem ativa) registrou o auge metamórfico colisional no domínio interno há 563 Ma. A migração das nappes Araçuaí contra o domínio cratônico, com o pico térmico metamórfico há 530 Ma, reflete a superposição de eventos colisionais múltiplos no Sistema Orogênico Mantiqueira. O embasamento da borda cratônica foi regenerado no Cambriano e engajado ductilmente no orógeno. Cavalga o domínio de foreland, no Quadrilátero Ferrífero, caracterizado por sistema thin-skinned de cavalgamento segmentado por domos do embasamento e calhas sinformais das supra-crustais. A colisão oblíqua entre o Sistema Orogênico Mantiqueira e o protocontinente consolidado pelo Orógeno Tocantins Meridional foi responsável pelo metamorfismo facies xisto-verde baixo das bacias sucessoras e migrou para norte, de 555 a 500 Ma. A colagem orogênica brasiliana, no sul do Cráton do São Francisco, representou a interação entre etapas orogênicas curtas em um longo processo de convergência, consumo de placas e migração de orógenos até o limite Cambro-Ordoviciano. O último evento metamórfico pré-colagem no embasamento cratônico ocorreu entre 2,03 - 2,06 Ga provavelmente sob regime extensional.

Active or passive continental margin? Geochemical and Nd isotope constraints of metasediments in the backstop of a pre-Andean accretionary wedge in southernmost Chile (46°30′-48°30′S)

Abstract Provenance analysis of siliciclastic sedimentary rocks gives indications of the tectonic evolution and setting of source regions and the rocks contained in them. The composition of sedimentary rocks ideally reflects the nature of these regions, and only indirectly the tectonic setting of the basin where the erosional debris is deposited. This makes it possible to interpret Late Devonian to Early Carboniferous metasedimentary basement rocks of the Andes in southernmost Chile as having been deposited at a passive margin, despite geochemical indications of an active margin setting for the source rocks, and the position of the metasediments in the backstop of an accretionary wedge. Major and trace elements point to felsic source rocks from an active margin environment. The Nd model ages of 1170–1490 Ma indicate that the source rocks were part of an old continental crust in the Late Palaeozoic. The ɛ Nd (T) values range between −7 and −2. These characteristics, in combination with the regional geology, suggest that the geochemical signal is dominated by rocks formed at an active margin, which later acted as feeders for the sediments deposited in a passive-margin environment. If corroborated by research in progress this emphasizes the problem of deducing the tectonic setting of a depositional basin from provenance data.

Late Neoproterozoic crustal growth in the European Variscides: Nd isotope and geochemical evidence from the Sierra de Córdoba Andesites (Ossa-Morena Zone, Southern Spain)

Because the Hercynian overprint was extremely weak, the Sierra de Córdoba (southeastern Ossa-Morena Zone, OMZ) provides an excellent opportunity to study the tectonic evolution of sequences deposited close to the Late Neoproterozoic–Early Palaeozoic boundary. In order to put constraints on the sources and geodynamic significance of the Late Proterozoic magmatism, a representative set of 18 igneous rocks, and 3 interbedded sedimentary rocks from the San Jerónimo Formation have been studied for major and trace element geochemistry and for the Sm–Nd isotopic systematics. The igneous rocks are generally porphyritic to microporphyritic andesites, with abundant plagioclase (±amphibole) phenocrysts. With the exception of two intrusive rocks, possibly not related to the Late Proterozoic episode, all the samples display positive ε Nd 550 Ma values, ranging from +2.9 to +7.6. Most of them, with +4< ε Nd 550 Ma <+6, exhibit LREE enrichment, high La/Nb ratios, and elevated Zr/Nb ratios ranging from 21 to 32. There is no obvious correlation between the shape of REE patterns, La/Nb ratios and ε Nd 550 Ma values, precluding simple models of late-stage interaction with typical crustal components having low ε Nd and high LREE/HREE and La/Nb ratios. Based on their major element composition and enriched, continental crust-like trace element characteristics, combined with distinctly positive ε Nd initial values, the Córdoba andesites document an episode of crustal growth through the addition of calc-alkaline magmas, extracted from a mantle reservoir which was strongly depleted in LREE on a time-integrated basis. The occurrence of interlayered sediments of continental provenance (negative ε Nd values) does not favour a purely ensimatic arc setting, remote from continental land masses, for this subduction-related magmatism, but the geochemical data suggest an active margin environment located on relatively juvenile crust. In any case, the Córdoba andesites document the addition of materials chemically similar to the bulk continental crust which were extracted from mantle sources with strong time-integrated LREE depletion. Therefore, they provide evidence for crustal growth related to Cadomian orogenic events during Late Proterozoic times.

Rhodope and Vardar: the metamorphic and the olistostromic paired belts related to the Cretaceous subduction under Europe

The Rhodope massif of Bulgaria and Greece is a complex of Mesozoic synmetamorphic nappes stacked in an Alpine active margin environment. A new analysis of the Triassic to Eocene history of the Vardar suture zone in Greece discloses its Cretaceous setting as a subduction trench. We present a geological traverse that takes into account these new observations and runs from the Hellenides to the Balkans, i.e. from the African to the Eurasian sides of the Tethys ocean, respectively. The present review first defines the revisited limits of the Rhodope metamorphic complex. In particular, the lower part of the Serbo-Macedonian massif is an extension of the Rhodope units to the west of the Struma river. Its upper part is separated as the Frolosh greenschist unit, which underlies tectonic slivers of Carpathic-Balkanic type. Several greenschist units, which locally yield Mesozoic fossils, follow the outer limits of the Rhodope. Their former attribution to a stratigraphic cover of the Rhodope has been proven false. They are divided into roof greenschists, which partly represent an extension of the Strandza Jurassic black shales basin, and western greenschists, which mostly derive from the Vardar Cretaceous olistostromic assemblage. The Rhodope complex of synmetamorphic nappes includes Continental Units and Mixed Units. The Continental Units comprise quartzo-feldspathic gneisses in addition to thick marble layers. The Mixed Units comprise meta-ophiolites as large bodies or small knockers. They are imbricated, forming an open dome whose lower, Continental Unit constitutes the Drama window. The uppermost Mixed Unit is overlain by remnants of the European plate. The presentday structure results from combined large-scale thrust and exhumation tectonics. Regional inversions of synmetamorphic senseof-shear indicate that intermediate parts of the wedge moved upward and forward with respect to both the lower and upper plates. A kinematic model is based on buoyancy-driven decoupling at depth between subducted continental crust and the subducting lithosphere. Continuing convergence allows coeval underthrusting of continental crust at the footwall, decoupling at depth, and upward-forward expulsion of a low-density metamorphic wedge above. The continental crust input and its upward return may have lasted for at least the whole of the Early Cretaceous, as indicated by isotopic ages and the deformation history of the upper plate. A Late Eocene marine transgression divides the ensuing structural and thermal evolution into a follow-up uplift stage and a renewed uplift stage. Revision of the limits of the Vardar belt in Greece first resulted in separating the Paikon mountain as a tectonic window below the Vardar nappes. It belongs to the western, Hellenic foreland into which a system of thrust developed downward between 60 and 40 Ma. The eastern limit is a dextral strike-slip fault zone that developed greenschist facies foliations locally dated at 50-40 Ma. Revision of the lithological components discloses the preponderance of Cretaceous volcano-detritic and olistostromic sequences that include metamorphite blocks of Rhodope origin. Rock units that belong to the Vardar proper (ophiolites, Triassic and Jurassic radiolarites, remnants of an eastern Triassic passive margin) attest for a purely oceanic basin. The Guevgueli arc documents the Jurassic change of the eastern Triassic passive margin into an active one. This arc magmatic activity ended in the Late Jurassic and plate convergence was transferred farther northeast to the subduction boundary along which the Rhodope metamorphic complex formed. We interpret the Rhodope and the Vardar as paired elements of a Cretaceous accretionary wedge. They document the tectonic process that exhumed metamorphic material from under the upper plate, and the tectonic-sedimentary process that fed the trench on the lower plate. The history of the Rhodope-Vardar pair is placed in the light of the history of the Tethys ocean between Africa and Europe. The Cretaceous subduction then appears as the forerunner of the present Hellenic subduction, accounting for several shifts at the expense of the lower plate. The Late Eocene shift, at the closure of the Pindos basin, is coeval with the initiation of new uplift and magmatism in the Rhodope, which probably document the final release of the low-density, continental root of the Rhodope from subduction drag.

Rhodope and Vardar: the metamorphic and the olistostromic paired belts related to the Cretaceous subduction under Europe

The Rhodope massif of Bulgaria and Greece is a complex of Mesozoic synmetamorphic nappes stacked in an Alpine active margin environment. A new analysis of the Triassic to Eocene history of the Vardar suture zone m Greece discloses its Cretaceous setting as a subduction trench. We present a geological traverse that takes into account these new observatons and runs from the Hellenides to the Balkans, i.e. from he African to the Eurasian sides of the Tethys ocean, respectively. The present review first defines the revisited limits of the Rhodope metamorphic complex. In particular, the lower part of the Serbo- Macedonian massif is an extension of the Rhodope units west of the Struma river. Its upper part is separated as the Frolosh greenschist unit, which underlies tectonic slivers of Carpathc-Balkanic type. Several greenschist units which locally yield Mesozoic fossils, follow the outer limits of the Rhodope. Their former attribution to a stratigraphic cover of the Rhodope has been proven false. They are divided into roof greenschists, which partly represent an extension of the Strandza Jurassic black shales basin, and western greenschists, which mostly derive from the Vardar Cretaceous olistostromic assemblage. The Rhodope complex of synmetamorphic nappes includes Continental Units and Mixed Units. The Continental Units comprise quartzo-feld-spathic gneisses in addition to thick marble layers. The Mixed Units comprise meta-ophiolites as large bodies or small knockers. They are imbricated, forming an open dome whose lower, Continental Unit constitutes the Drama window. The uppermost Mixed Unit is overlain by remnants of the European plate. The present-day structure results from combined large-scale thrust and exhumation tectonics. Regional inversions of synmetamorphic sense-of-shear indicate that intermediate parts of the wedge moved upward and forward with respect to both the lower and upper plates. A kinematic model is based on buoyancy-driven decoupling at depth between subducted continental crust and the subducting lithosphere. Continuing convergence allows coeval underthrusting of continental crust at the footwall, decoupling at depth, and upward-forward expulsion of a low-density metamorphic wedge above. The continental crust input and its upward return may have lasted for at least the whole of the Early Cretaceous, as indicated by isotopic ages and the deformation history of the upper plate. A Late Eocene marine transgression divides the ensuing structural and thermal evolution into a follow-up uplift stage and a renewed uplift stage. Revision of the limits of the Vardar belt in Greece first resulted in separating the Paikon mountain as a tectonic window below the Vardar nappes. It belongs to the western, Hellenic foreland into which a system of thrust developed downward between 60 and 40 Ma. The eastern limit is a dextral strike-slip fault zone that developed greenschist facies foliations locally dated at 50–40 Ma. Revision of the lithological components discloses the preponderance of Cretaceous volcano-detritic and olistostromic sequences that include metamorphite blocks of Rhodope origin. Rock units that belong to the Vardar proper (ophiolites, Triassic and Jurassic radiolarites, remnants of an eastern Triassic passive margin) attest for a purely oceanic basin. The Guevgueli arc documents the Jurassic change of the eastern Triassic passive margin into an active one. This arc magmatic activity ended in the Late Jurassic and plate convergence was transferred farther northeast to the subduction boundary along which the Rhodope metamorphic complex formed. We interpret the Rhodope and the Vardar as paired elements of a Cretaceous accretionary wedge. They document the tectonic process that exhumed metamorphic material from under the upper plate, and the tectonic-sedimentary process that fed the trench on the lower plate. The history of the Rhodope-Vardar pair is placed in the light of the history of the Tethys ocean between Africa and Europe. The Cretaceous subduction then appears as the forerunner of the present Hellenic subduction, accounting for several shifts at the expense of the lower plate. The Late Eocene shift, at the closure of the Pindos basin, is coeval with the initiation of new uplift and magmatism in the Rhodope, which probably document the final release of the low-density, continental root of the Rhodope from subduction drag.

EVOLUÇÃO TECTÔNICA DO CINTURÃO DOM FELICIANO NO ESCUDO SUL-RIO-GRANDENSE: PARTE II - UMA CONTRIBUIÇÃO A PARTIR DAS ASSINATURAS GEOFÍSICAS

The recognition of three geophysical domains has permitted a re-evaluation of the geotectonic models proposed for the precambrian rocks of the Sul-rio-grandense shield. The eastern, central and western geophysical domains are separated by important gravimetric and aeromagnetic discontinuities and consist of assemblages of rocks with petrogenetic characteristics of specific tectonic environments. The eastern domain includes two neoproterozoic calc-alkaline plutonic sequences, typical of active margin environments, and developed upon reworked eoproterozoic basement. It is separated from the central domain by the Porto Alegre Suture, which is marked by a gravimetric and aeromagnetic anomaly. The central compartment is sub-di vided into two segments by the Cacapava Suture, marked by an aeromagnetic anomaly. Its central-eastern segment consists of tectonically reworked eoproterozoic basement rocks and the central-western segment is composed of juvenile neoproterozoic crust. It is separated from the western geophysical compartment by the Sao Gabriel Suture, that is the boundary between the Rio de La Plata Craton to the west and the Dom Feliciano Belt, to the east. The geophysical data ha ve allowed a more precise definition of the nature and boundaries of some of the petrotectonic assemblages recognised, in addition to making it possible to test and discuss the tectonic models proposed for the Sul-rio-grandense shield from a methodological and epistemological point of view

Continental Slope Deposits from a Late Cretaceous, Tectonically Active Margin, Southern California

ABSTRACT Extensive, active-margin continental-slope deposits are not commonly preserved in the rock record. The middle and upper portions of the Holz Shale (Ladd Formation) in Black Star Canyon, northern Santa Aria Mountains, represent such a sequence where shale and mudstone strata are dissected by large, coarse-grained channel-fill deposits. Channels preserve evidence of filling primarily by conglomeratic debris flows, high-density turbidites, and classic Bouma low-density turbidites; slumping and traction-current mechanisms were less important. Associated with channels are pebbly mudstones, poorly developed levee facies, and submarine chute and gully deposits. Interbedded turbidites, contourites, and hemipelagic sediments dominate interchannel strata. Hemipelagic sediments exhibit sedimento ogic fabrics that range from biologically dominated (homogeneous, bioturbated) to physically dominated (fine-scale, planar-laminated, anaerobic). This variation in fabric as well as associated digenetic information indicates that the anaerobic zone, although generally at some depth below the sediment-water interface, at times migrated up into the overlying water column. Foraminiferal assemblages within hemipelagic sediments are dominated by agglutinated forms which indicate deposition at bathyal depths. Macroinvertebrate evidence includes 1) the interchannel paleocommunity, dominated by the bivalve Inoceramus and the deposit-feeding trace fossil Chondrites and 2) the submarine-channel paleocommunity, comprised mainly of the trace fossils Thalassinoides and Ophiomorpha. Previous studies have demonstrated that these Late Cretaceous active-margin environments included a narrow continental shelf. Abundance of terrestrial plant material, paucity of displaced shelf faunas, well-rounded conglomerate clasts, and the coarse-grained texture of these deposits suggest that one or more of the Holz Shale submarine channels were receiving sediment directly from terrestrial environments. Paleogeographic reconstructions have postulated Cenozoic lateral and rotational displacement of associated Upper Cretaceous strata in the Transverse Ranges of southern California. Paleocurrent and other sedimentologic analyses of Holz Shale strata support these reconstructions and suggest that a submarine-fan complex, which was fed by Holz Shale submarine channels, is presently located to the north in the Simi Hills and Santa Monica Mountains.

Macroborings and Epizoans on Late Cretaceous Rudists from West Coast Active-Margin Environments: ABSTRACT

Rudists are a major component of Cretaceous carbonate strata. Studies of rudists from these strata have demonstrated that epizoans are sparse but macroborings are relatively common. Previous studies have not been made of macroborings and epizoans on rudists from nearshore, elastic, active-margin environments. Beds of the Rosario Formation lower sandstone containing the late Campanian rudist Coralliochama orcutti crop out along the northern coast of Punta Banda on the Pacific coast of Baja California, Mexico. Serial sections of rudist skeletons from this area show that borings include Entobia and varying forms of Trypanites, while epizoans include serpulid worms, the bivalve Acila and juvenile Coralliochama. Macroborings occur on 30% of examined specimens, primarily on the upper portion of the attached valve and on the free valve, while epizoans are found on 10% of examined specimens, typically from the middle third to the commissure of the attached valve and on the free valve. Only a small portion of the skeleton may be covered by epizoans, and some boring and encrusting appears to have oc urred during life. Inhibition by rudists of epizonas, a concept proposed by Kauffman and Sohl, probably was not as effective in Coralliochama as in rudists from Antillean reefs. In addition, Coralliochama appears to be more heavily bored than Antillean reef rudists. These results aid in understanding the broad environmental range of Late Cretaceous rudists and the adaptive response to this range. Rudists of clastic environments may have been more susceptible to boring and encrustation than those from reef associations. End_of_Article - Last_Page 452------------

Paleoenvironmental Analysis of Late Cretaceous Active Margin Continental Slope Deposits, Southern California: ABSTRACT

Extensive active margin, continental slope and upper fan deposits are not commonly preserved in the rock record. The Holz Shale (Ladd Formation, Black Star Canyon, Santa Ana Mountains) represents such a sequence where shale and mudstone strata are dissected by numerous coarse-grained channel-fill deposits. Channels preserve evidence of filling primarily by conglomerate debris flows, high-density turbidites, and classic Bouma low-density turbidites; slumping and traction-current mechanisms were less important. Associated with channels are submarine chutes, pebbly mudstones, and poorly developed levee facies. Interbedded turbidites, contourites, and hemipelagic sediments dominate interchannel strata. Hemipelagic sediments exhibit sedimentologic textures that range from biol gically dominated (homogenous, bioturbated) to physically dominated (fine-scale, planar-laminated, anaerobic) fabrics. This variation in texture and associated diagenetic information indicates that the anaerobic/aerobic boundary was generally at some depth below the sediment-water interface, but at times migrated up into the overlying water column. Foraminiferal assemblages within hemipelagic sediments are dominated by agglutinated forms which indicate deposition at bathyal depths. Macroinvertebrates include (1) the interchannel paleocommunity, dominated by the bivalve Inoceramus and the deposit-feeding trace fossil Chondrites, and (2) the submarine channel paleocommunity, comprised mainly of the trace fossils Thalassinoides and Ophiomorpha. Previous studies have demonstrated that these active margin environments included a narrow continental shelf. Abundance of terrestrial plant material, paucity of displaced shelf faunas, well-rounded conglomerate clasts, and the coarse-grained texture of these deposits suggest that one or more of the Holz Shale submarine channels was receiving sediment directly from terrestrial environments. End_of_Article - Last_Page 458------------

Quaternary Sedimentation and Tectonics on Oregon Continental Slope--Active-Margin Environment: ABSTRACT

Quaternary Sedimentation and Tectonics on Oregon Continental Slope--Active-Margin Environment: ABSTRACT