Middle Bathyal Depths Research Articles

Evolution of the Southwest Australian Rifted Continental Margin During Breakup of East Gondwana: Results From International Ocean Discovery Program Expedition 369

AbstractInternational Ocean Discovery Program Expedition 369 drilled four sites on the southwestern Australian continental margin, in the deep water Mentelle Basin (MB) and on the neighboring Naturaliste Plateau (NP). The drillsites are located on continental crust that continued rifting after seafloor spreading began further north on the Perth Abyssal Plain (PAP) between magnetochrons M11r and M11n (133–132 Ma), ending when spreading began west of the NP between chrons M5n and M3n (126–124 Ma). Drilling recovered the first in situ samples of basalt flows overlying the breakup unconformity on the NP, establishing a magnetostratigraphically constrained eruption age of >131–133 Ma, and confirming a minimal late Valanginian age for the breakup unconformity (coeval with the onset of PAP seafloor spreading). Petrogenetic modeling indicates the basalts were generated by 25% melting at 1.5 GPa and a potential temperature of 1380°C–1410°C, consistent with proximity of the Kerguelen plume during breakup. Benthic foraminiferal fossils indicate that the NP remained at upper bathyal or shallower depths during the last 6 Myr of rifting and for 3–5 Myr after breakup between India and Australia. The limited subsidence is attributed to heat from the nearby Kerguelen plume and PAP spreading ridge. The margin subsided to middle bathyal depths by Albian time and to lower bathyal (NP) or greater (MB) depths by late Paleogene time. Periods of rapid sedimentation accompanied a westward jump of the PAP spreading ridge (108 Ma), rifting on the southern margin (100–84 Ma), and opening of the southern seaway between Australia and Antarctica (60–47 Ma).

Cenomanian to Coniacian sea-level changes in the Lower Benue Trough (Nkalagu Area, Nigeria) and the Eastern Dahomey Basin: palaeontological and sedimentological evidence for eustasy and tectonism

Abstract The Benue Trough formed in close relation to the opening of the South Atlantic and experienced sea-level fluctuations of different magnitudes during the Cenomanian to Coniacian interval. We identify depositional environments from outcrop sections and a drilling as control record. Lines of evidence for the interpretation include facies analyses, foraminiferal assemblage composition (P/B-ratio) and the presence of planktonic deep-water indicators. While the analysis of the well data from the Dahomey Basin indicates a continuous deep-water (bathyal) environment, the succession in the Nkalagu area of the Lower Benue Trough evolved in a different and more complex way. Beginning with latest Cenomanian shoreface to shelf deposits, a long period of subsidence lasted until the middle Turonian when pelagic shales and calcareous turbidites were deposited at upper to middle bathyal depths. These conditions continued during late Turonian and Coniacian times. The general deepening trend of the Lower Benue Trough was mainly controlled by tectonic subsidence and was superimposed by eustatic sea-level changes, resulting in periodically changing palaeowater depths. We were able to identify eight sea-level rises and falls that can be attributed to 405 kyr eccentricity cycles. The amplitudes of the sea-level changes were most likely in the range of several tens to a few hundred metres. The deposition of carbonate turbidites at Nkalagu was probably triggered by eustatic sea-level lowstands.

An eddy, a wake and a plume: controls on bathyal foraminifera around Tobago, western tropical Atlantic Ocean

Oceanic islands in the paths of currents induce the development of wakes and stationary eddies. The situation to the lee of Tobago, western tropical Atlantic Ocean, is further complicated by the occurrence of the seasonally variable, hypopycnal Orinoco plume. Here we investigate the impact of the combined plume, wake and eddy on bathyal benthic foraminifera to the NW of Tobago. Three surface sediment samples were recovered from around each of five well-sites to the NW of Tobago, three of the sites (Warap-A, Cassra-A and Cassra-CC) being at upper bathyal depths and two (Bene-1, Sancoche-1) at middle bathyal depths. Warap-A, Cassra-A, Cassra-CC and Bene-1 form a transect along the northern side of the leeward wake, while the other two sites are in the vicinity of the stationary eddy. The samples obtained around Sancoche-1 were taken north of the wake. These were supplemented by samples from four 80-cm piston cores from upper bathyal and outer neritic depths sampled at ~ 10 cm intervals. Benthic foraminifera reveal different biofacies at upper (Warap-A, Cassra-A, Cassra-CC) and middle (Bene-1, Sancoche-1) bathyal depths. The upper bathyal biofacies is dominated by Cassidulina curvata and the middle bathyal biofacies contains abundant Uvigerina hispidocostata, both of which are indicative of a high nutrient flux. The presence of Martinottiella communis and M. pallida at Warap-A indicate that pore waters are low in dissolved oxygen in the immediate lee of the island. Percentages of the fauna as serial tests indicated decreasing current velocities with increasing depth, as confirmed by the high abundance of Cibicides ex gr. aknerianus in the shallowest water core. Upper bathyal bottom-current strength was at its lowest in the immediate lee of the island. Species indicative of a perennial nutrient flux were more abundant to the NW, where the interaction of the plume and eddy appears to concentrate nutrients. The short cores, each from a different biofacies, indicate that these environmental conditions have been in place for at least the later Holocene. The most northerly, upper bathyal core presented a stable community structure with low assemblage turnover, while two cores taken farther south (upper bathyal and outer neritic) had an expansive structure with high assemblage turnover. These data raise the possibility of using benthic foraminifera to track the positions of the plume, core and eddy throughout the later Neogene.

Reconciling regional continuity with local variability in structure, uplift and exhumation of the Timor orogen

Along-strike variations in orogenic development can be difficult to constrain. Resulting assumptions projecting similarity or variability along strike can lead to erroneous conclusions at the orogen scale. Young orogens provide opportunities to document limits of along-strike projection and test factors that may control lateral variability. Here we present new constraints on the history of uplift, exhumation and shortening in the Timor orogen from West Timor, Indonesia. Structural mapping documents a foreland thrust stack of Jurassic-Miocene Australian margin strata and a hinterland antiformal stack of Permo-Triassic Australian continental units duplexed below Banda Arc lithosphere. Biostratigraphy within the piggyback Central Basin reveals earliest deepwater synorogenic deposition at 5.57–5.53Ma, uplift from lower to middle bathyal depths at 3.35–2.58Ma, and uplift from middle to upper bathyal depths at 2.58–1.30Ma. Hinterland Permo-Triassic strata yield apatite (U-Th)/He ages of 0.33–2.76Ma, apatite fission track ages of 2.19–3.53Ma and partially reset zircon (U-Th)/He ages. These thermochronology ages are youngest or most strongly reset in the center of the antiformal stack and yield modeled exhumation rates of 0.45–3.31km/Myr. A balanced cross section reveals a minimum of 300km of shortening including 210km of Australian continental subduction below the Banda forearc. When compared to published results from Timor-Leste, these data show that the timing of initial collision, synorogenic basin uplift, and total shortening amount were broadly similar across the island. Therefore, despite along-strike changes in orogen morphology and significant small-scale spatial variability in deformation, first-order structural similarity dominates at large scales. We suggest that along-strike variations in orogen morphology are due to changes in the distribution of deformation within the orogen driven by differences in backstop strength, internal wedge strength and basal décollement friction as well as the presence of the wedge-top Central Basin in West Timor.

Neogene history of the Carapita Formation, Eastern Venezuela Basin

Based on the stratigraphic distribution of planktonic microfossils (mostly planktonic foraminifera) and abundance patterns of benthic foraminifera we determine the temporal completeness of one land section and three wells through the Lower and Middle Miocene Carapita Formation of eastern Venezuela, compare the extent of the hiatuses in the sections and document changes in paleodepth at these localities during the early and Middle Miocene. We determine that changes in paleodepth are associated with hiatuses, but see no relationshipwith the global changes in sea-level inferred from deep sea isotope records. This strongly suggests that there was a strong tectonic forcing on stratigraphic architecture at upper and middle bathyal depths, as to be expected in a tectonically active area. However, similar stratigraphic patterns are also observed elsewhere, implying that a widespread tectonic structuring of the stratigraphic architecturemay have been operative. Recognition of hiatuses (not merely unconformities) as primary stratigraphic components will make possible an uninterrupted documentation of sequences boundaries from subaerial to bathyal environments, and help determine objectively the structural mechanism(s) operating on the genesis of stratigraphic sequences (sensu Catuneanu et al. 2009). Biostratigraphy (and biochronology) are the main tools to understanding this structuring.

Long-term subduction-erosion along the Guatemalan margin of the Middle America Trench

A new analysis of Deep Sea Drilling Project (DSDP) Leg 84 data demonstrates that the dominant process controlling the Guatemala margin tectonic evolution since ca. 25 Ma is subduction-erosion. Data from benthic foraminifera, assemblages from upper-slope DSDP Sites 568, 569, and 570 indicate long-term, progressive subsidence from upper to middle bathyal depths (600–1000 m) ca. 19 Ma to modern abyssal depths (>2000 m). Rapid subsidence migrated landward starting at the Oligocene-Miocene boundary time under the current middle slope, where it increased sharply ca. 19 Ma, reached the current upper slope by ca. 15 Ma, and arrived at the uppermost slope ca. 2 Ma. Subsidence indicates crustal thinning by basal tectonic erosion of mass from the underside of the upper plate. Under the assumption that, in the Miocene, the morphology of the forearc was similar to that of today, landward migration of the trench was at a rate of 0.8–0.9 km/m.y. This linear rate corresponds to a tectonic erosion rate of the submerged forearc of 11.3–13.1 km3·m.y.−1·km−1. The evolution of arc magmatism and superfast spreading at the East Pacific Rise since early Miocene time may have caused slab shallowing and tectonic erosion that readjusted the forearc geometry.

Biostratigraphy, Paleoenvironments and Stratigraphic Evolution of the Neogene of St. Croix, U.S. Virgin Islands

Analysis of the microfauna of the Jealousy Formation and Kingshill Limestone from four cored wells, two water wells, and ten outcrops in the Kingshill basin of St. Croix allows a comprehensive reconstruction of the Neogene stratigraphic evolution of the island. This study results in the first precise biostratigraphic zonation and paleoenvironmental evaluation of these Neogene units in the subsurface as well as a re-evaluation of the age and correlation of outcropping strata. The Jealousy Formation is an entirely subsurface unit of dark, plankton-rich marls which contains planktonic foraminifera indicative of the lower part of the middle Miocene (Praeorbulina glomerosa Zone to the Globorotalia fohsi fohsi Zone) and a middle bathyal benthic microfauna. The Kingshill Limestone conformably and diachronously overlies the Jealousy Formation and is subdivided into two members, both of which occur in outcrops and the subsurface. The lower of these, the La Reine Member, is characterized by buff pelagic limestones and marls with an upward increasing proportion of intercalated shelf-derived debris. It ranges from the basal part of the middle Miocene (Praeorbulina glomerosa Zone) to the upper part of the uppermost Miocene (Globorotalia humerosa Zone), and exhibits a transition from middle bathyal (600-800m) to upper bathyal (200-300m) environments. The uppermost part of the Kingshill Limestone, the Manning's Bay Member, is made up of skeletal debris-rich carbonate slope deposits and is placed near the Miocene-Pliocene boundary. Biostratigraphic control indicates activation of the St. Croix graben fault system and formation of the Kingshill basin at the beginning of the late Miocene. The Neogene section records deposition in an open-marine setting at middle bathyal depths during the early middle Miocene, shoaling gradually to upper bathyal depths in the late middle and late Miocene and more rapidly to a shallow-marine setting at the beginning of the Pliocene. Shoaling was accompanied by an increasing proportion of transported shallow marine sediments. Subsidence analysis for the section indicates more than 400 m of tectonic uplift between 10.5 and 3.5 Ma. Two depositional shifts in the Kingshill Limestone may be tied chronostratigraphically to postulated major global sea-level falls. An interval of carbonate debris in the type section of the Kingshill Limestone may record a fall at 10.5 Ma, and a disconformity at the base of the Mannings Bay Member may be tied to a fall at 5.5 Ma. INTRODUCTION St. Croix is the southernmost of the U.S. Virgin Islands, located on the crest of an east-west oriented submarine platform near the eastern end of the northern Caribbean plate boundary zone. The central plain of the island is underlain by a northeast-southwest trending sedimentary trough (Gerhard et al. 1978; Whetten 1966), here referred to as the Kingshill basin (text-fig. 1), and bounded by the mountainous East End and Northside Ranges. Most studies of the Kingshill basin have been based on outcrop data and considered the Tertiary carbonate succession to record deposition in an isolated graben system (Whetten 1966; Multer et al. 1977; Gerhard et al. 1978; Lidz 1984). These strata have been assigned a wide range of ages in past studies, ranging from middle Oligocene (Vaughan 1923) to early Pliocene (Lidz 1982) (Table 1). However, exposures are limited in number and predominantly represent the upper part of the section. A drilling program undertaken in the mid-1980's has allowed us to conduct a more comprehensive study of the Neogene stratigraphy of the island and better constrain the nature and timing of the tectonic events affecting the evolution of the Kingshill basin. This study focuses on foraminiferal and ostracode faunas present in cores collected during this drilling program and in outcrop samples collected during field studies. The goals of this study are to: (1) to establish an accurate age and environmental framework for the Neogene strata of St. Croix; (2) to resolve surface-subsurface correlations of these strata; and (3) to outline the stratigraphic evolution of the Kingshill basin and its implications for the Neogene tectonics of the structurally complex region at the eastern end f the northern Caribbean plate boundary zone.

Deep‐sea echiurans: possible producers of Zoophycos

Kotake, N. 1992 07 15: Deep-sea echiurans: possible producers of Zoophycos. The trace fossil Zoophycos in the upper Pliocene deep-sea deposits (Shiramazu Formation) distributed in the southernmost part of Boso Peninsula, central Japan, appears to be a product of feeding and excretory behavior by surface deposit-feeders inhabiting middle bathyal depths. Ecology and anatomy of modern deep-sea echiurans are consistent with those of the Zoophycos producers based on the Pliocene examples. Consequently, deep-sea echiurans may be producers of Zoophycos. Systematic lamellae structures of the spreite reflect excretory behavior in sediment resulting from deposit-feeding on the sea-floor. Zoophycos, deep-sea echiurans, Pliocene, Boso Peninsula, Japan.

Neogene stratigraphy, paleoceanography and paleobiogeography in northwest South America and the evolution of the Panama seaway

The Atrato basin in the pacific coastal region of NW South America (Colombia) exhibits stratigraphic and biostratigraphic characteristics similar to other Neogene Pacific coastal basins of southern Central America and northern South America. An evaluation of the Neogene stratigraphy and foraminiferal biostratigraphy with emphasis on paleoceanographic and paleobiogeographic phenomena indicate the following. (1) Previous to the middle Miocene, well aerated deep and open oceanic conditions, and free and active water circulation prevailed along the steep continental margins of NW South America. (2) During the early middle Miocene, right after the Neogene Hiatus NH 2 (15.2 Ma), tectonic disturbances triggered the initial uplift of the Panama sill. Bottom water circulation, foraminiferal and sedimentation changes are observed in both the Pacific and Caribbean coastal areas of NW South America. (3) During the middle Miocene, coinciding with the Neogene Hiatus NH 3 (12.9–11.8 Ma), an abrupt paleobathymetric change from lower to middle bathyal depths indicates a major uplift of the Panama sill to about 1000 m, reflecting the middle Miocene tectonic disturbances in NW South America. (4) During the late middle Miocene, immediately following this uplift a distinctive Pacific benthic foraminiferal fauna abruptly appeared and extended from Ecuador to California. Because these assemblages do not occur in the adjacent Caribbean region, a circulation barrier between the Atlantic and Pacific Oceans as a result of intensification of the cool, marginal California Current is proposed. Anoxic middle to upper bathyal environments prevailed under well aerated cool surface waters in the Pacific coastal areas of NW South America. (5) During the latest Miocene, immediately following the Neogene Hiatus NH 6 (7.0–6.3 Ma), surface water circulation between the Caribbean and the Pacific was re-established and the previous influence of the California Current disappeared. A rapid filling and shallowing of the basin to mostly neritic depths is indicated along the Pacific and Caribbean coastal basins of Colombia as well as a progressive increase in similarity between the coastal Pacific and Caribbean benthic biotas. This shallowing in the early late Miocene, also appears to have been associated with the earliest terrestrial interchange of ground sloths to North America and raccoons and their allies to South America. (6) Immediately following early Pliocene time, the Panamanian isthmus became completely emergent providing a terrestrial environment favourable for intermingling of terrestrial faunas and floras between North and South America. I correlate this Pliocene uplift with the Neogene Hiatus NH 8 (3.7-3.1 Ma). Four regional unconformities associated with tectonic disturbances in the NW corner of South America, have also been recognized and correlated with Neogene hiatuses recorded from the oceans: early-middle Miocene, middle Miocene, late Miocene and early Pliocene.

Middle Miocene planktonic microfossils and lower bathyal foraminifera from offshore southern California

Lower bathyal sediments representing portions of the Luisian and Mohnian stages of Kleinpell (1938) occur on a submarine ridge near Tanner Bank, offshore southern California. The presence of abundant and well-preserved calcareous nannofossils, diatoms, silicoflagellates, radiolarians and foraminifera allows accurate correlations with the onshore type sections of these stages. In terms of the calcareous nannofossil zones, the age range is from the Sphenolithus heteromorphus Zone to the Discoaster kugleri Zone. Although abundant benthic foraminifera indicative of the Luisian and Mohnian are present, they are accompanied by species more characteristic of the Pliocene Repettian Stage of Natland (1952) and the Pliocene-Miocene “Delmontian” Stage of Kleinpell (1938). Many of these latter species live today at lower bathyal depths (below 2,000 m), others occur in lower bathyal sediments as old as Oligocene, but are absent in the onshore type sections of the Luisian and Mohnian stages in coastal California. We ascribe their absence in onshore sequences to deposition at middle bathyal depths. The known chronostratigraphic ranges of several species are extended and five new species and two new subspecies of benthic foraminifera are described.The following new taxa are described: Bolivina pelita n. sp., Cassidulinella inflata n. sp., Globocassidulina undulata n. sp., Cibicidoides mckannai miocenicus n. subsp., C. mckannai sigmosuturalis n. subsp., Pullenia fragilis n. sp., Parafissurina inornata n. sp.

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

Paleo-Oceanographic Significance of Eocene Diatomites in Kreyenhagen Formation of California: ABSTRACT

Diatomaceous shales of the Kreyenhagen Formation underlie much of the western San Joaquin valley of California and contain a remarkable record of middle and late Eocene paleo-oceanographic events along the evolving continental margin of this region. The Kreyenhagen Formation is similar in many respects to the much studied Miocene Monterey Formation of California with both units constituting known or potential sources of mature hydrocarbons due largely to their unusual biogenic character and similar but not identical depositional histories. Lithofacies and biofacies patterns within the Kreyenhagen indicate that it was deposited at upper and middle bathyal depths with the occurrence of laminated diatomites marking the intersection of a well-developed oxygen minimum layer ag inst continental slopes and basin sills analogous to Holocene continental margin settings off Peru, California, and in the Gulf of California. Quantitative analysis of benthic Forminifera within a 565 ft (172 m) thick sequence of Kreyenhagen shale in the Tumey Hills area of Fresno County illustrates that three low diversity - high abundance biofacies, dominated respectively by Bulimina corrugata, B. microcostata, and Bolivina spiralis, are indicative of various slope and base-of-slope environments characterized by differing levels of dissolved oxygen as dictated by the thickness and intensity of Eocene oxygen minima. Moreover, Bolivina spiralis appears to be homeomorphic with Suggrunda eckisi, a Neogene species with an apparent affinity for oxygen minimum facies. The sequential appearanc of massive and laminated diatomites and associated low oxygen megafaunal and microfaunal associations within the Kreyenhagen Formation allows estimates to be made of dissolved oxygen values in the Eocene water masses which, in turn, imply variations in climatically tuned rates of upwelling, siliceous productivity, and mid-water circulation as a function of the deteriorating global climate during middle and late Eocene time. Finally, the biogenic character of the Kreyenhagen diatomites within a Paleogene stratigraphic sequence otherwise dominated by terrigenous clastics suggests that deposition of this unit was coincident with a period of reduced flux of diluting terrigenous sediment to the continental margin likely induced by eustatic and/or tectonic adjustment of the Eocene strandline. /P> End_of_Article - Last_Page 1695------------

Paleoecologic Evaluation of Late Eocene Zonations of West Coast: ABSTRACT

Investigations of the late Eocene benthic forams along the West Coast indicate that the current zonal schemes can be modified to accommodate the constraints imposed on the organisms by ecologic conditions and thus become less provincial. The benthic foraminiferal assemblages of northwestern Oregon and southwestern Washington are used to develop a series of ecologic facies indicative of bathymetry and/or water mass. Upper depth limits, trends, clines, and morphologic variations of this group provide tools for determining ecology. The faunas of the type sections of the California and Washington stages, zones, and subzones, when analyzed in terms of this ecologic model illustrate some of the deficiencies inherent in these schemes. The late Eocene zones of California have a s rong association with depth; that is, Narizian zones are lower or middle bathyal whereas Refugian zones are outer neritic or upper bathyal. The late Eocene zones of Washington are diagnostic of middle bathyal depths with considerable transport; as a result partial rather than total ranges are used in the development of these zones. Although no new zones are proposed, it is possible to revise the existing zones to recognize the total ranges and bathymetric or other ecologic controls. The late Narizian Stage encompasses a bathyal and neritic facies. The bathyal facies is correlative with a modified Bulimina corrugata Zone of California and the Uvigerina cf. U. yazooensis Zone of Washington. The neritic late Narizian facies corresponds to a modified Bulimina schencki-Plectofrondicularia cf. P. jenkinsi Zone of Washington and a modified Amphimorphina jenkinsi Zone of California. The Refugian Stage can also be divided into a neritic and bathyal facies. However, the early and late subdivision of this stage is weak. The Refugian is equivalent to the modified versions of the Cibicides haydoni Subzone, Uvigerina atwilli ubzone, and the Uvigerina vicksburgensis Zone of California and a modified Sigmomorphina schencki Zone of Washington. The Cibicides haydoni Subzone is the neritic facies of the Refugian whereas the faunas of the Uvigerina atwilli Subzone, Uvigerina vicksburgensis Zone, and Sigmomorphina schencki Zone represent the bathyal Refugian facies. End_of_Article - Last_Page 494------------

Biostratigraphy and Paleoecology of Early Miocene Through Early Pleistocene Benthonic and Planktonic Foraminifera, San Joaquin Hills-Newport Bay, Orange County, California: ABSTRACT

A uniquely complete and continuous sequence of early Miocene through early Pleistocene marine sediments is exposed in Bonita Canyon on the western flank of the San Joaquin Hills and at adjacent Newport Bay, California. These sediments are assigned to the early to middle Miocene Topanga Formation, middle to late Miocene Monterey Shale, late Miocene to early Pliocene Capistrano Formation, and early Pliocene to early Pleistocene Fernando Formation. The total thickness of the sequence studied is more than 2,400 m. Quantitative analysis of benthonic foraminiferal biofacies indicates that: (1) the lower parts of the Topanga Formation were deposited at inner to outer shelf depths, (2) upper Topanga and Monterey Shale diatomaceous sediments were deposited at upper to middle bathyal depths within a closed basin where ambient water contained less than 1 ml/l of dissolved oxygen, (3) lower bathyal depths marked by abundant radiolarian tests occurred during deposition of the Capistrano Formation, and (4) the Fernando Formation was deposited at lower bathyal through outer shelf depths. Stratigraphic variations of gross faunal parameters including foraminiferal number, radiolarian number, planktonic-benthonic ratio, and percent of displaced benthonic species provide additional quantitative evidence of th shelf-to-basin-to-shelf paleobathymetric history of this sequence. Ranges of individual species of benthonic Foraminifera allow the Saucesian, Relizian, Luisian, Mohnian, Repettian, Venturian, Wheelerian, and Hallian Stages to be recognized. Cool to warm temperate planktonic foraminiferal biofacies dominated by Globigerina concinna sl. and G. angustiumbilicata occur in the Topanga Formation and lower Monterey Shale. Temperate planktonic assemblages dominated by G. bulloides sl. dominate in the upper Monterey Shale and parts of the Fernando Formation. A subarctic biofacies containing sinistral populations of G. pachyderma is present at discrete intervals within late Miocene, middle and late Pliocene, and early Pleistocene sediments, whereas a subtropical-warm temperate biofacies dominate by Globoquadrina dutertrei is restricted to the early Pliocene. Ranges of critical planktonic Foraminifera including Globorotaloides trema, Globorotalia mayeri, G. menardi, G. crassaformis, G. inflata, G. truncatulinoides, Orbulina universa, Sphaeroidinella dehiscens, and the radiolarian Prunopyle titan provide additional criteria for age assignment and correlation with the paleomagnetic-radiometric time scale. End_of_Article - Last_Page 434------------

Base of Pleistocene in Los Angeles Basin, California: ABSTRACT

Populations of Globigerina pachyderma (Ehrenberg) are consistently left-coiling, cool-water types in sections of the lower Pleistocene of the Los Angeles basin, California; right-coiling, warm-water populatons are predominant in the upper Pliocene. The base of the Pleistocene, defined in this way, indicates a major change in water masses of the southern California area at that time. Recent radiometric dates by Obradovich place this boundary about 3 m.y. ago. Populations of Globigerina pachyderma reflect bathyal conditions generally; modern populations in End_Page 604------------------------------ typical form live suspended deeper than about 50-100 m. Basing the beginning of the Pleistocene on planktonic foraminiferal criteria, benthic Foraminifera indicate that outer-shelf conditions prevailed at the beginning of the Pleistocene in some areas, upper-bathyal depths existed in other places, middle-bathyal depths in some areas, and lower-bathyal depths at still other locations. Thus, a boundary based on upper limits of benthic species (Epistominella pacifica and Uvigerina peregrina), and defined by comparing sections from different parts of the Los Angeles basin, differs hundreds of meters in stratigraphic position. End_of_Article - Last_Page 605------------

Paleoecologic, Sedimentary, and Structural History of the Late Tertiary Capistrano Embayment, California: ABSTRACT

The Capistrano Embayment comprises a distinct geologic unit of the southern California area. Paleobathymetry, sediments, and microfaunas within the embayment indicate that it has had a different structural history, different from the Los Angeles Basin on the north. Marine invasion of the trough began in the Paleocene and ceased in the Pleistocene. This report develops a detailed history of the embayment from middle Miocene to late Pliocene time using data from two well exposed sections within the boundaries of the embayment. Analogy between ecologic niches of living benthonic Foraminifera and fossil forms encountered indicates that middle bathyal depths were attained by the middle Miocene. By late Pliocene the trough was filled to shelf depths. Repetitive changes in morphology of benthonic species provides additional evidence of varying paleobathymetry. Variation in abundance of cool and warm-water planktonic Foraminifera indicates three periods of distinctly cool surface temperature between late Miocene and late Pliocene time. Increase in radiolarian diameter provides evidence for surface temperatures in sediments barren of Foraminifera. Peak radiolarian abundance suggests that the deepest point in basinal evolution existed in the early Pliocene at which time water depth neared 1,750 meters. Correlation of the two sections is based on: (a) a Mohnian horizon of Globigerina pachyderma which coil sinistrally, (b) the uppermost point of abundant radiolarian tests, (c) the uppermost point of the radiolarian Prunopyle titan, and (d) the horizon of peak radiolarian number. These mutually corroborative planktonic criteria demonstrate the time transgressive relationship of existing stages based on benthonic Foraminifera. Repettian faunas, for example, appear much earlier in the rapidly filling southern end of the embayment than in the northern deeper area. A restricted, oxygen-deficient, closed-basin system, characterized by laminated diatomaceous sediments, originated during a period of early Miocene diastrophism. Closed-basin conditions allowed only a marginal benthonic foraminiferal fauna to exist, analogous to the existing fauna of the oxygen deficient Santa Barbara basin. Restricted basin plain conditions prevailed until the end of the Miocene. Instantaneously deposited coarse sediments (turbidites) emphasize pulses of structural activity in the middle Miocene, late Miocene, and middle Pliocene. Sediments provide evidence of a landmass to the west of the embayment. Turbidites are recognized on the basis of displaced Foraminifera, plant material, and sedimentary structures. Paleoecologic and sedimentary analyses delineate a characteristic basin-filling sequence in the Capistrano Embayment, similar to events taking place today in the Gulf of California and off southern California. End_of_Article - Last_Page 361------------

Miocene-Pliocene Paleoecology of San Fernando Basin, California: ABSTRACT

Sedimentary rocks exposed along the periphery of the San Fernando Valley indicate the area was a separate marine basin during Miocene and Pliocene time. Although the surrounding geology is well known, the basin is virtually unrecognized as a major Tertiary basin of Southern California. Structure, stratigraphy, and sedimentology indicate the basin's history is similar to that of other Continental Borderland basins. Localized subsidence in the Late Middle Miocene formed the basin as a discrete unit. Basin filling took place during Pliocene and Pleistocene time. Benthonic Foraminifera indicate that the San Fernando basin was separate from the adjacent and deeper Ventura basin but remained an integral part of the east-west Ventura embayment. Over 900 meters of Late Miocene and Pliocene sediments are well exposed on the south side of the basin. Shales and diatomites typify the Miocene sequence whereas silts and sands are characteristic of the Pliocene. Laminated diatomite was probably deposited in a subsill oxygen-deficient environment analogous to the existing Santa Barbara basin. Coarse, arkosic continental sands interfinger with Pliocene marine sediments at the eastern end of the basin. Benthonic Foraminifera and Radiolaria show that End_Page 1771------------------------------ subsidence to middle bathyal depths occurred during Late Miocene (Delmontian) and Early Pliocene (Repettian) time. Rapid shoaling during the Pliocene is evidenced by the systematic appearance of slope, shelf, and neritic foraminiferal faunas. Abundant Pliocene macrofossils also characterize shelf-depth deposits. Shallow-water micro and macrofaunas within deep water sediments are interpreted as displaced by turbidity currents or slumping. Planktonic Foraminifera and an increase in radiolarian (Spumellina) diameter suggest cool surface temperatures during the Late Miocene and increasingly warmer temperatures in the Early Pliocene. End_of_Article - Last_Page 1772------------