Luisian Stage Research Articles

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.

Point Arena Coccolith Correlations with Mid-Tertiary California Stage Stratotypes: ABSTRACT

Previous provincial California stage designations for the Iversen-Skooner Gulch-Gallaway-Point Arena sequence do not appear to be the correct geologic age for these rocks in the Point Arena area of northern California. Time-transgressive pseudo-Saucesian benthic foraminiferal assemblages show little change in their gross composition through much of the deep-water Gallaway-Point Arena sequence near Point Arena, which includes middle Miocene strata. About 30 mid-Tertiary coccolith taxa were recovered from 120 localities near Point Arena from Iversen-Skooner Gulch-Gallaway-Point Arena strata. The first occurrence (biohorizon) of Sphenolithus heteromorphus marks the base of the Helicosphaera ampliaperta Zone within the middle Gallaway Formation and serves as an important datum for correlation with California stage stratotype localities in central and southern California. The Iversen Basalt appears to span the Oligocene-Miocene boundary. Revised radiometric dates for the Iversen Basalt indicate an average age of 23.8 Ma which is younger than the world Oligocene-Miocene boundary estimate of 24.6 Ma. Fossiliferous interbeds believed to occur within the lower part of this volcanic unit at inland localities contain the highest occurrence of Dictyococcites bisectus and D. scrippsae. The stratigraphic distribution of coccolith taxa and the recognition of the Sphenolithus heteromorphus datum suggest that the Skooner Gulch and lower to middle Gallaway sequence is the early Miocene equivalent of the type Saucesian of Los Sauces Creek; that the middle and upper Gallaway and lower Point Arena Formations are the late early Miocene equivalents of the type Relizian section at Reliz Canyon; and that part of the Point arena Formation is equivalent in age to the early middle Miocene type Luisian Stage. End_of_Article - Last_Page 1695------------

Diatom Biostratigraphy and Paleoecology of the Type Section of the Luisian Stage, Central California

Diatom Biostratigraphy and Paleoecology of the Type Section of the Luisian Stage, Central California

Microfossils from Type Relizian and Luisian Stages of California: ABSTRACT

Kleinpell designated the strata of the Monterey Formation in Sec. 28, T28S, R14E, as the type locality of the Luisian Stage. Microfossils from this section, however, were never described; instead, the Luisian Stage was characterized by foraminiferal faunas in strata from Reliz Canyon that overlie strata of the type Relizian Stage. The two Relizian and three Luisian Zones described by Kleinpell can be recognized in the benthic foraminiferal faunas from recently collected samples from Reliz Canyon. This sequence of zones cannot be recognized in benthic foraminiferal faunas of the type Luisian section or in the nearby Wilson's corner section, partly because the Reliz Canyon faunas represent deeper and more diverse faunas than those from the type Luisian area. In the Reliz Canyon section, benthic foraminiferal faunas diagnostic of the Saucesian-Relizian boundary coincide with the base of the Helicosphaera ampliaperta Zone. The Relizian-Luisian boundary as indicated by the benthic foraminifers occurs near the H. ampliaperta-Sphenolithus heteromorphus boundary. Poor preservation and limited exposures prevent a more definitive calibration. Occurrence of late Luisian benthic foraminifers with meager floras containing Cyclogargolithus floridanus suggests an age no younger than the lower part of the Discoaster exilis Zone. In the type Luisian section, benthic foraminifers diagnostic of the Luisian Stage occur both above and below strata designated by Kleinpell as type Luisian. Nannofossil floras in the lower part of the type section are indicativ of the S. heteromorphus Zone and diatoms diagnostic of the Denticulopsis lauta Zone and the lowermost part of the D. hustedtii-D. lauta Zone also are present. Sparse radiolarian faunas are dominated by long-ranging species, typical of present-day California-current faunas. End_of_Article - Last_Page 957------------

Planktonic Calibration of Provincial Miocene Stages of California: ABSTRACT

Age assignments and correlations of Miocene marine strata in California are usually made with reference to a sequence of benthic foraminiferal stages. During the past decade refined calcareous and siliceous planktonic microfossil biochronologies have been developed that provide a means for calibrating the California provincial stages to international standards while also testing the chronostratigraphic reliability of correlations derived by equating stage diagnostic benthic foraminifer assemblages in different areas. Diatoms are particularly useful for dating the upper, silica-rich part of the Monterey Formation whereas calcareous nannofossils are useful for dating the lower part of the Monterey Formation and older Miocene units. Data from type sections show that the base of the Saucesian Stage is near the Oligocene-Miocene boundary of international usage, the base of the Relizian Stage falls at the base or in the lower part of the early Miocene Helicosphaera ampliaperta Zone (nannofossil), and the Luisian Stage essentially coincides with the middle Miocene Sphenolithus heteromorphus Zone (nannofossil) and the Denticulopsis lauta Zone to Subzone a of the Denticulopsis hustedtii--D. lauta Zone (diatom). The top of the type Mohnian Stage is within the latest Miocene to early Pliocene Nitzschia reinholdii Zone (diatom), whereas the type section of the Delmontian Stage is within Subzones b and c of the middle Miocene Denticulopsis hustedtii-D. lauta Zone. The type Delmontian is therefore equivalent to the lower Mo nian of most California sections. Compilation of data from throughout California indicates some overlap in time of benthic foraminifer assemblages diagnostic of the Zemorrian and Saucesian Stages and of the Relizian and Luisian Stages. The transition from Luisian to Mohnian benthic foraminifer assemblages, however, consistently occurs near the top of the Sphenolithus heteromorphus Zone and within Subzone a of the Denticulopsis hustedtii-D. lauta Zone. End_of_Article - Last_Page 974------------

Santa Monica and Santa Ana Mountains--Relation to Oligocene Santa Barbara Basin

An analysis of the stratigraphy and structure of the Santa Monica and Santa Ana Mountains demonstrates a close affinity between the two, one that can be explained only by a much closer original geographic relation of the two ranges. The Santa Monica Mountain block, bounded by the Malibu Coast fault on the south and the Las Posas fault on the north, began a westward movement across the ancestral basin during the Luisian stage of the middle Miocene. Approximately 33 mi (53 km) of left-lateral displacement took place prior to the Mohnian stage of the upper Miocene, breaking the original basin into the separate Ventura and Los Angeles basins joined by a San Fernando subbasin. This displacement took place in no more than 2.5 m.y. at a minimum rate of movement of 0.8 in. (2 cm) year. Four mi (6.4 km) of post-Mohnian left-lateral displacement subsequently occurred. The magnitude of displacement is determined by projecting the Santa Ana Mountains parallel with the San Andreas direction, to a point of intersection with the Raymond fault, and then by matching similar stratigraphy on opposite sides of the fault. Restoration of the Santa Monica Mountains block to its pre-Luisian location, combined with the addition of known thicknesses of sections of the Sespe Formation, permits the construction of original Sespe isopachs and the reconstruction of the Oligocene basin, herein called the Santa Barbara basin. This basin was bounded, south of the present Ventura basin and west of the present Los Angeles basin, by a positive Franciscan area, the Anacapia of Reed and Hollister. The Luisian orogeny, responsible for the westward rifting of the Santa Monica Mountains, also produced the breakup of Anacapia, and exerted the most profound influence on southern California geology of any Cenozoic event.

Late Miocene and Early Pliocene Correlations in California Province: ABSTRACT

The late Miocene of California was divided into four Oppelian zones by Kleinpell. He grouped these zones into larger time-stratigraphic units, the Mohnian and Delmontian Stages. As a discipline for these zones and stages, Kleinpell used benthonic foraminifers and larger invertebrates in continuous stratigraphic sequences. The Bolovina modeloensis, Bulimina uvigerinaformis, and the Bolovina hughesi Zones represent the Mohnian, and the Bolovina obliqua Zone represents the lower half of the Delmontian Stage. No foraminiferal zone was designated for the upper half of the Delmontian Stage. These two stages and four foraminiferal zones can be found in continuous stratigraphic (superpositional) relation with demonstrable middle Miocene, late Luisian, faunas below and early Plioc ne faunas above in the following areas: Palos Verdes Hills, Huasna-Nipomo, Monterey, San Pablo Bay area. Additional continuous stratigraphic sections representing the middle Miocene and early late Miocene, the Luisian and Mohnian Stages, are found in Newport Beach; the Luisian, Mohnian, and Delmontian Stages are found in the Naples Bluff section; the Mohnian and Delmontian Stages are found in Woodland Hills (type Mohnian); the Delmontian and early Pliocene are found in the Reef Ridge area. All of these stratigraphic sections contain one or more of the following groups of microfossils: diatoms, radiolarians, silicoflagellates, and planktonic foraminifers. These microfossil groups, together with the benthonic foraminifers and larger invertebrates corroborate Kleinpell's original zonal sequ nce. End_of_Article - Last_Page 440------------

Late Paleogene-Neogene Planktonic Biostratigraphy and Its Geologic Implications, California: ABSTRACT

Planktonic microfossils indicate that at least in some typical sections, the Narizian Stage (Kreyenhagen Shale) is middle Eocene, spanning part of the range for the P10-P14 zones (about 49-45 m.y. ago). The Refugian Stage is essentially late Eocene, representing Paleogene zones P15-P17 (about 45-37.5 m.y. ago). Oligocene time is represented approximately by the Zemorrian Stage, defined by Paleogene zones P18-P22 (about 37.5-22.5 m.y. ago). Miocene subdivisions are: early Miocene, Saucesian Stage (Neogene zones N4-N8), radiometric age span about 23-15 m.y. ago; middle Miocene, Relizian, and Luisian Stages (Neogene zones N9-N12), radiometric age span about 15-11 m.y. ago; and the late Miocene Mohnian Stage (Neogene zones N13-N18), radiometric age span about 11-3 m.y. ago. T e Pliocene is represented by the Repettian, Venturian, and the lower part of the Wheelerian Stage (Neogene zones N19-N21) with a radiometric age span of about 3-1.79 m.y. ago. The Quaternary is younger than 1.79 m.y., embracing the upper part of the Wheelerian Stage, the Hallian, and younger units, representing Neogene zones N22 and N23. Employing planktonic criteria, boundaries of stages based on benthic species are clearly time-transgressive. A major structural change occurred in the Zemorrian Stage, the Oligocene, introducing a new set of basin patterns with the destruction of the Farallon plate and the adjacent spreading center, about 29-24 m.y. ago. A second important structural change occurred about 4 m.y. ago, in the late Miocene, with the development of a different pattern of basins and the formation of the Gulf of California. This new tectonic pattern resulted from, or was related to, the development of a westerly motion of the Pacific plate. The major volcanism of the California Tertiary occurred following the destruction of the Farallon plate, during the early and middle Miocene. End_Page 429------------------------------ Maximum sedimentation rates for the middle Eocene Narizian Stage (Kreyenhagen Shale) are less than 100 m/m.y., for the late Eocene (Refugian Stage) about 100 m/m.y., for the Oligocene Zemorrian Stage about 166 m/m.y., for the early Miocene about 140 m/m.y., for the middle Miocene about 500 m/m.y., for the late Miocene about 250 m/m.y., and for the Pliocene-Pleistocene they are almost 2,000 m/m.y. The very rapid sedimentation rates, following the second major change in tectonic patterns about 4 m.y. ago, are similar to those today in Santa Barbara basin; they are less than the much higher rates recognized for the late Pleistocene in basins of the continental borderland. End_of_Article - Last_Page 430------------

Luisian and Mohnian Biostratigraphy of Monterey Shale at Newport Lagoon, Orange County, California: ABSTRACT

Sediments of Luisian and Mohnian age are exposed almost continuously in a 1,400-ft section of Monterey Shale along the margin of Newport Lagoon. The lowermost 200 ft appears to be barren of fossils, consequently, no definite age assignment can be demonstrated for this part of the section. Overlying this barren interval is approximately 265 ft assigned to the Luisian Stage on the basis of occurrence of Brizalina advena striatella, Brizalina imbricata, Pullenia miocenica, Siphogenerina spp. and Valvulineria californica californica. Approximately 40 ft of section directly above the Luisian sediments is assigned to the Brizalina modeloensis Zone of the lower Mohnian Stage as defined by the restricted occurrence of the nominate species (sensu stricto); also present, but not restricted to the zone, are Bulimina unigerinaformis, Cassidulina monicana, and Concavella gyroidinaformis. Overlying the Brizalina modeloensis Zone is 325 ft of sediments End_Page 438------------------------------ which is assigned to the Bulimina uvigerinaformis Zone of the lower Mohnian Stage. The Bulimina uvigerinaformis Zone can be readily divided into an older subzone designated the Concavella gyroidinaformis Subzone and a younger subzone here designated the Brizalina woodringi Subzone. The top of the older subzone is marked by the last occurence of Concavella gyroidinaformis; the Concavella gyroidinaformis Subzone is approximately 210 ft thick at this locality. The overlying Brizalina woodringi Subzone is based on the continued occurrence of Bulmina uvigerinaformis after the extinction of Concavella gyroidinaformis. This subzone is approximately 115 ft thick with its top, and the top of the lower Mohnian Stage, marked by the last occurrence of Bulimina uvigerinaformis. Above the lower Mohnian Stage at Newport Lagoon is approximately 470 ft of foraminiferal Monterey Shale assigned to the upper Mohnian Stage. Among the many species present are Bolivina hughesi, Brizalina benedictensis, Brizalina decurtata, Brizalina giradensis, Brizalina granti, Cassidulinella renulinaformis and Discorbinella valmonteensis. No foraminifers were found in the uppermost 100 ft of Monterey Shale which overlies the sediments of demonstrable Mohnian age. Cyclococcolithina neogammation, a fossil coccolith, was found to have its last occurrence coincident with the top of the lower Mohnian Concavella gyroidinaformis Subzone at Newport Lagoon. Another coccolith, Reticulofenestra pseudoumbilica, was observed to have its first occurrence at the base of the lower Mohnian Brizalina modeloensis Zone. The last occurrence of Sphenolithus heteromorphus was confirmed to be in the uppermost Luisian Stage. End_of_Article - Last_Page 439------------

Petrology and Possible Tectonic Significance of Late Cenozoic Volcanic Rocks, Southern California and Baja California

The continental borderland and adjacent land areas of southwestern California, Baja California, and the Gulf of California area were the site of extensive Cenozoic volcanism. Radiometric dates and stratigraphic relations indicate major mid-Miocene volcanism (Jacumba, California basalts 18.7 ± 1.3 m.y.; Tijuana area of Mexico, 14.3 ± 2.6 m.y.; Channel Islands and Los Angeles basin area, California ~15.5 m.y., Luisian stage). High-alumina basalts of both tholeiitic and alkalic types are predominant; andesites and dacites are less voluminous but are locally important. These silicic rocks appear to be differentiates of the basalts. A Pliocene episode of volcanism is characterized by abundant alkalic basalts and low-alumina tholeiites and by lesser amounts of high-alumina tholeiite (Santa Rosa basalts, Murietta, California, 8.3 ± 0.5 m.y.). Scattered outpourings of alkalic and tholeiitic basalt, andesite, and rhyolite continued in the region until latest Pleistocene time. The mid-Miocene volcanism appears to be synchronous with the postulated initial stages of crustal dilational which formed the Gulf of California. All of the late Cenozoic volcanism shows close spatial relations to fault zones of the area, and a genetic relationship between faulting, regional dilation and volcanism is inferred. The compositional trend of the sequence of volcanic rocks over the past 20 m.y. may be generalized as follows: (1) high-alumina basalts of both alkalic and tholeiitic types, (2) high-alumina tholeiite, (3) low-alumina alkalic basalt, (4) low-alumina tholeiite. Andesites and dacite appear as differentiates of steps (2) and (4). The complete sequence is not represented in any one area but may apply to the whole region. The sequence suggests a progressive increase to the depth of magma separation in the mantle and may be related to the progressive change in thermal gradients and in crustal and mantle stresses as the borderland was disrupted.

Tropical Planktonic Zones and Calcareous Nannoplankton Correlations in Part of the California Miocene

THE material used in this study is from exposures along the east shore of Newport Bay, California (Fig. 1). This section contains good assemblages of benthonic foramini-fera of Kleinpell's upper Luisian and lower Mohnian stages1. The Luisian and Mohnian stages are represented by the upper middle and the lower upper part of the Miocene Monterey Shale. These stages are traditionally denned by their characteristic benthonic foraminiferal species. In recent years emphasis has been placed on worldwide planktonic foraminiferal zonation and correlation. Calcareous nannoplankton have also proved to have great biostratigraphic value because of their extreme abundance in many samples, wide geographic distribution and the very short geological range of some species.

Correlation of Marine Middle Tertiary Stages of California with Tropical Planktonic Zones: ABSTRACT

The Oligocene-Miocene boundary is equated with End_Page 467------------------------------ the Globigerinoides trilobus datum of the tropics; this is approximately equivalent to the base of the Saucesian Stage of California. In ascending order, the lower Miocene Catapsydrax dissimilis, C. stainforthi, and Globigerinatella insueta zones correlate with the Saucesian and basal Relizian Stages. Assemblages characterizing the Catapsydrax stainforthi zone have been identified in the upper part of the lower Saucesian Stage of Reliz Canyon, California. Further, Sphenolithus belemnos is a good index of the lower Miocene of tropical areas and of the Saucesian Stage of California. The orbuline datum of the tropics, which is in the uppermost part of the Globigerinatella insueta zone, is the base of the middle Miocene. This level appears to correspond to the base or the lower part of the Relizian Stage of California. In ascending order, the middle Miocene Turborotalia peripheroronda and Globorotalia peripheroacuta zones correlate with the Relizian Stage, and the Globorotalia praefohsi and Globorotalia fohsi zones correlate with the Luisian Stage. Sphenolithus heteromorphus occurs in the Relizian Stage and also in the Globigerinatella insueta and Turborotalia peripheroronda zones of the tropics. Discoaster kugleri is an index of the uppermost zone of the G. fohsi sequence and it is identified in the upper Luisian Stage of Newport Bay, California. Orbulines first a pear in the Luisian Stage in California. The Globorotalia menardii datum is the base of the upper Miocene in tropical areas, and it corresponds approximately to the Globigerina pachyderma datum of California or the base of the lower Mohnian. The dextral Globigerina pachyderma zone of the lower Mohnian contains Catinaster coalitus and Discoaster bollii, which are indices of the Turborotalia mayeri and Globorotalia menardii zones of the tropics. The sinistral Globigerina pachyderma zone of the upper Mohnian and basal Delmontian Stages represents the first major cool event in the middle Neogene; it correlates with the Turborotalia acostaensis (Globigerina bulloides) zone of tropical areas and with the sinistral G. pachyderma zone of the Messinian of Italy. The base of the Pliocene is marked by the appearance of Sphaeroidinella dehiscens and by the change from sinistral to dextral populations of Neogloboquadrina dutertrei. End_of_Article - Last_Page 468------------

Middle Tertiary Planktonic Foraminiferal Facies, San Joaquin Basin, California: ABSTRACT

Biofacies analyses of the middle Tertiary marine units of the San Joaquin basin, California, show that the most continuous deep-water areas were near the southern end of the basin and close to the San Andreas fault system which bounds the basin on the west. Isopach maps demonstrate that the major sediment accumulation was generally in the more rapidly subsiding and deeper water areas of the basin, suggesting that mechanisms such as turbidity currents, sand flow, etc., must have been important in the deposition there. Planktonic foraminiferal facies are restricted to the southern part of the basin. Minor planktonic facies occurred in the Zemorrian and Saucesian Stages, perhaps 28 to about 20 million yr B.P. A major expansion of planktonic facies occurred during the Relizian Stage, about 20 to perhaps 17 million yr B.P. Only minor planktonic faunas are identified with the Luisian Stage, about 17 to perhaps 14 million yr B.P. A second major expansion of planktonic faunas occurred during the Mohnian Stage, perhaps 14 to about 11 million yr B.P. After the Mohnian, the deeper oceanic connection was attenuated rapidly and planktonic faunas disappeared in the latest Miocene. Abundance variations and distribution patterns of middle Tertiary planktonic foraminiferal facies of the San Joaquin basin indicate that (1) there was an important deep-water opening toward the west across the San Andreas fault system, (2) other possible connecting channels must have been of a much smaller magnitude, allowing no major influx of oceanic planktonic populations, (3) the main distribution pattern is counterclockwise indicating a similar counterclockwise current pattern around the southern boundary of the basin, (4) there were two major expansions End_Page 518------------------------------ of the current system and planktonic facies, and (5) the shallow-water facies west of the San Andreas fault at the south end of the San Joaquin Valley are anomalous. The facies anomaly across the San Andreas fault is consistent with the concept of major right lateral-slip movement since the middle Tertiary. End_of_Article - Last_Page 519------------

Review of Tertiary Stratigraphy and Foraminiferal Zonations of Western Washington State: ABSTRACT

Tertiary marine sediments and volcanic rocks ranging in age from early Eocene to late Miocene, or possible Pliocene, are present in southwestern Washington State. Total thickness of these rocks is more than 50,000 feet. The Eocene history of the area is characterized by widespread vulcanism. Sedimentary rocks definitely dated as early Eocene have not been found in southwestern Washington. Middle Eocene rocks are represented by volcanics and interbedded sediments of the Crescent Formation and typically contain Amphistegina california and Asterigerina crassiformis. Upper Eocene rocks are represented by the marine Raging River Formation and the non-marine Puget Group in the eastern part of the area. To the southwest, equivalents of these formations range from marine to non-marine facies and are mapped as McIntosh, Northcraft, Skookumchuck, and Cowlitz Formations. Further west, upper Eocene rocks are wholly marine and are mapped as the Cowlitz Formation. Total thickness of middle and upper Eocene rocks i at least 15,000 feet. Foraminiferal zones equivalent to Laiming's B Zones and B-1-A, A-2, and A-1 Zones of the California Eocene can be identified. In early Oligocene time, regional subsidence, accompanied by notable pyroclastic vulcanism, took place and Oligocene rocks are characteristically tuffaceous. Rocks assigned an Oligocene age have a maximum thickness of 9,000 feet in southwestern Washington and comprise the Lincoln Formation. Two lithologic members usually are distinguished: a lower basaltic sandstone member and an upper tuffaceous siltstone member. Three foraminiferal zones are recognized and are correlated with the Refugian and Zemorrian stages of California. In early Miocene time, deposition continued uninterrupted in the same basins that were receiving Oligocene sediments. In middle Miocene time widespread diastrophic activity was accompanied by withdrawal of the seas from southwestern Washington, except along the present Pacific Ocean front and a deep embayment in the Grays Harbor-Montesano area. In the Chehalis basin, Miocene sediments consist of 2,000 feet of wholly non-marine rocks comprising the Astoria (?) Formation (early Miocene) and Wilkes Formation (late Miocene). In the Grays Harbor-Montesano basin, lower and middle Miocene rocks consist of an estimated 6,000 feet of marine sediments mapped as the Astoria Formation. The Astoria Formation is subdivided informally into three lithologic units which are assigned an early and middle Miocene age, equivalent to the Saucesian, Relizian, and possibly Luisian stages of California. At Ocean City, sediments notably different from the typical Astoria Formation are correlated tentatively with the lower portion of the Astoria Formation. Marine sediments of late Miocene to questionably Pliocene age occur only in the Grays Harbor-Montesano basin and the Pacific Ocean coastal strip. In the Grays Harbor-Montesano basin, these rocks are mapped as Montesano Formation and consist of 2,500 feet of marine conglomerate, sandstone, and siltstone containing Foraminifera which indicate a late Miocene age equivalent to the Mohnian and Delmontian stages of California. Along the Pacific Coast, in the vicinity of Ocean City, sediments of equivalent age are represented by up to 6,000 feet of sediments informally denominated End_Page 1080------------------------------ the Ocean City and Quillayute Formations. A late Miocene age for these rocks is supported by the presence of Rotalia garvyensis, Uvigerina hootsi, and Pulvinulinella gyroidinaformis. End_of_Article - Last_Page 1081------------

Orthodontia for the general practitioner: P. N. Williams (New York). The Dental Digest, July, 1925, xxxi, 7

Outcrop sections of the lower Monterey Formation in Laguna Hills, southern California, con-tain an unusual early middle Miocene (Luisian Stage) subtropical marine fauna of macroinverte-brates and microfossils. The basal exposures consist of patch reefs constructed by serpulids, oys-ters, and bryozoans interbedded with sands, silts, and muds dominated by bivalves. In the overlying section, turbidite sands yield abundant ostracodes. The purpose of this study is to analyze this ostracode assemblage in relation to the paleoenvironment of deposition.Ten genera and 14 new species comprise the ostracode assemblage recovered at one of the locali-ties where the turbidites had been exposed: Microcytherura sp. (33%), Aurila sp. A (27%), Aurila sp. B (11%), Aurila sp. C (7%), “Cytheretta” sp. (5%), Loxoconcha (Loxoconcha) sp. (4%), Am-bostracon sp. (3%), Loxoconcha (Palmoconcha) cf. L. (P.) polugari Finger (3%), Paracosta cf. P. huddlestoni Finger (3%), Hamanella cf. H. implexa Finger (2%), Neonesidea sp.? (<1%), Cythe-relloidea sp. (<1%), “Microcytherura” sp. (<1%), and Xestoleberis sp. (<1%). The species of Loxoconcha (Palmoconcha), Paracosta, and Hamanella may be descendants of those previously described from the Oligo-Miocene Rincon Formation of central California. Rare specimens of other neritic taxa were obtained from nearby outcrops of this discontinuous unit.The ostracodes appear to be a mixed neritic assemblage, and most are preserved as complete carapaces, suggesting that they had been rapidly displaced and buried downslope. This mode of deposition is also evident in the associated foraminifers, which constitute a mixed assemblage of inner-neritic to upper middle-bathyal taxa. Mudstones within the superjacent section are barren of ostracodes but richer in bathyal foraminifers, suggesting that the embayment continued to get wider and deeper during the early middle Miocene.Rapid basin subsidence is a tectonic phenomenon prevalent throughout the early and middle Miocene of California. Further study of basal sedimentary units along the California borderland could uncover additional ostracode assemblages that can be useful in determining the geologic, ecologic, and paleontologic history of the East Pacific Rim.