Gerald A. Fowler

Oregon Continental Margin Structure and Stratigraphy: A Test of the Imbricate Thrust Model

The Cenozoic structural and stratigraphic framework of the Oregon continental margin records several intervals of significant tectonism (uplift) with subsequent erosion and truncation of older structures. During late Cenozoic time this framework displayed many of the characteristics of a fore-arc structure defined by Karig (1970). It is also characterized by a substantial amount of continental accretion, interpreted to be the result of underthrusting of the oceanic plate. The compressional thrust model (Seely, et al., this volume; Burk, 1968) offers the best explanation for the late Cenozoic evolution of the Oregon margin.

Planktonic Foraminiferan-Radiolarian Ratios and Holocene-Late Pleistocene Deep Sea Stratigraphy off Oregon

The ratio of planktonic forammiferans to radiolarians has proven to be an effective and easily determined parameter for the establishment of deep-sea faunal stratigraphy in the northeastern Pacific off Oregon and Washington. The Pleistocene-Holocenc boundary, dated at 12,500 years B.P., is marked by a change from a dominance of planktonic foramimferans below to a dominance of radiolarians above. Three less-well-defined intervals of increased radiolarian abundance occurred from 16,000 to 18,000, 25,000 to 28,000, and greater than 31,000 or 30,000 years B.P. These suggest slight warming periods during the late Pleistocene.

Uplift of the Continental Margin and Possible Continental Accretion off Oregon

Sedimentary rocks collected from the continental shelf and slope off the central coast of Orgeon contain fossil benthic foraminifers of Pliocene and Miocene age. These fossils indicate water depths much greater than those from which the rocks were collected, implying that the rocks have been uplifted as much as 1000 meters since their deposition. Uplift of this magnitude near the edge of the continent is interpreted as representing an early stage of continental accretion, possibly as a result of compression normal to the continental margin. The average maximum horizontal component of this accretion would be about 16 kilometers.

Holocene faunal stratigraphy and paleoclimatic implications of deep-sea sediments in Cascadia Basin

Abstract Hemipelagic sediments in Cascadia Basin were characterized by an abundance of planktonic foraminifers during glacial periods and by an abundance of radiolarians during interglacial periods. The relative abundances of these organisms are used to construct a paleoclimatic curve which includes several intervals indicative of cooling periods within the Holocene. These periods, from 5,000 to 4,000 and at 2,000 years B.P. appear to correlate with those reported in other paleoclimatic studies in widely separated localities.

Foraminiferal trends in a Holocene turbidite

Abstract Sediments cored in Cascadia Deep-Sea Channel in the northeastern Pacific show a rhythmic alternation of thick, graded, olive-green turbidity current deposits and thin layers of gray pelagic clay. Benthic foraminifera indigenous to shallow, intermediate and abyssal depths occur within a single graded layer from the channel floor. Forms from all depths occur throughout the layer indicating that the transporting turbidity current accumulated and mixed sediment as it flowed from the source area to the abyssal plain. A striking size sorting of foraminifera occurs along with the grading and sorting of the sediment itself, indicating deposition from suspension.

Tertiary Foraminiferal Paleoecology and Biostratigraphy of Part of Oregon Continental Margin: ABSTRACT

Several thousand feet of late Tertiary marine sediments crop out across Heceta Bank on the central Oregon shelf. Rock samples from two east-west profiles (44°05^prime and 44°10^prime N) have yielded large, well-preserved foraminiferal faunas. The stratigraphic sequence of the samples has been determined using sparker subbottom profiles. Two possible stratigraphic units are delineated on the basis of their foraminiferal content, seismic-reflection characteristics, and lithology. The older is characterized by Bolivina seminuda foraminata, B. semiperforata, B. spissa, Bulimina subacuminata, B. subcalva, Buliminella cf. B. exilis, Epistominella pontoni californica, and Uvigerina peregrina; less than 10 percent planktonic foraminifers; and right-coiling Globigerina pachyderma. This fauna represents paleodepths of 500-1,000 m and is dated as Pliocene. The younger unit is characterized by Cassidulina minuta, Eilohedra levicula, Epistominella exigua, Nonionella spp., Trifarina angulosa, and Uvigerina juncea; more than 50 percent planktonic foraminifers; and left-coiling G. pachyderma. This unit was deposited at depths of 100 200 m. This fauna could be Pliocene or Pleistocene. Paleoenvironmental data require minimal uplifts ranging from approximately 1,000 m for the oldest sampled strata to 100 m for the youngest. There was a general shoaling throughout the deposition of the units. End_of_Article - Last_Page 467------------

Foraminiferal Trends in Oregon Sublittoral: ABSTRACT

Benthic foraminiferal species from a part of the central Oregon shelf are grouped into four distinct bathymetric faunas. Elphidiella hannai and Buccella spp. are indicative of Fauna A (17-50 m). Fauna B (50-100 m) is characterized by Buliminella elegantissima and Elphidium magellanicum. In the rocky area off Cape Arago Cassidulina californica, C. limbata, Cibicides fletcheri, and C. lobatulus become dominant. Spiroplectammina biformis, Textularia earlandi, and Trifarina angulosa are important species of Fauna C (100-175 m). Abundant species of Fauna D (175-339 m) include Eggerella advena, Epistominella exigua, and Uvigerina juncea. Species diversity increases offshore to a maximum of about 35 benthic species near 100 m and then decreases slightly with depth. The standing crop is small nearshore, increases to a maximum of approximately 300 specimens per 20 cm3 between 125 and 150 m, and then declines. The total benthic population reaches a maximum of approximately 6,000 specimens per sample between depths of 150 and 175 m. Values then decrease to about 2,000 specimens at 339 m. A maximum of 15-20 percent live benthic specimens occurs near 50 m. Planktonic Foraminifera normally constitute less than 10 percent of the total population. Maximum percentages generally are in water shallower than 100 m, whereas maximum specimens per sample are at the deepest stations. Porcelaneous specimens do not exceed 6 pe cent of the benthic population. Agglutinated Foraminifera are more abundant than calcareous specimens at depths greater than 100 m. Miscellaneous biofacies trends indicate that: thecameobians are most abundant between 50 and 125 m; statoliths and otoliths are most abundant between 75 and 200 m; the largest number of ostracods are between 25 and 150 m; and radiolarians generally predominate over Foraminifera below 250 m. End_of_Article - Last_Page 466------------

Late Pleistocene Planktonic Foraminiferal Trends Off Oregon: ABSTRACT

Oscillations in radiolarian-planktonic foraminiferal ratios with increasing depth in several deep-sea sediment cores from the southern Cascadia basin and the adjoining Blanco fracture zone off Oregon are thought to be the result of climatic fluctuations. Greater production of planktonic foraminifers compared with radiolarians seems to have occurred during glacial advances of the Pleistocene, whereas radiolarians predominated during glacial retreats. At least four intervals are evident in the deep-sea sediment record of the last 50,000-70,000 years. Based on radiocarbon dates, these intervals correspond approximately with late Wisconsin glacial advances and retreats as defined for the Puget lowlands of Washington. Five species generally make up 90 per cent or more of the planktonic foraminiferal assemblage. Globigerina pachyderma and Globigerina bulloides together compose more than half of the assemblage. In grain-size fractions of less than 177 microns, Globigerina quinqueloba and Globigerinita uvula predominate. A few specimens of Globoquadrina hexagona, Globigerina digitata, and Globigerinoides ruber have been observed. The species composition of planktonic Formaminifera does not appear to reflect the change from glacial to post-glacial climates. Neither do the coiling habits of Globigerina pachyderma. This species is dominantly left-handed throughout the sections examined. Right-handed forms reach a maximum of 13 per cent and average 2.7. End_of_Article - Last_Page 464------------

Foraminiferal Paleoecology of Upper Miocene Montesano Formation, Western Washington: ABSTRACT

The Montesano Formation, which ranges in age from late Miocene to questionable early Pliocene, is exposed in an area of approximately 250 square miles of Grays Harbor County in western Washington. It averages 2,500 feet in thickness and consists predominantly of fine- to medium-grained sandstone, with mudstone, pebbly sandstone, and conglomerate locally significant. The formation possibly represents the last marine incursion in a depositional basin that existed generally as a strongly negative feature through most of the Tertiary. Paleoenvironmentally significant faunas from the Montesano Formation include: (1) rock-boring pelecypods, (2) Chione-Spisula molluscan assemblages, (3) a Miliammina fusca fauna, (4) a Buliminella elegantissima fauna, (5) a Nonionella fauna, (6) a Bolivina fauna, (7) a Uvigerina peregrina hispidocostata fauna, and (8) a Bolivina seminuda fauna. The succession of these assemblages, the associated quantitative microfaunal trends, and the sedimentary evidence indicate that the formation was deposited in a sea that first transgressed from west to east over Grays Harbor basin and then regressed. In the western part of the basin, water depths increased progressively from zero to more than 3,000 feet. On the east, deposition took place initially in the littoral zone, later the outer shelf, an finally under probable tidal sand-flat conditions. A local laminated mudstone unit contains an impoverished fauna suggestive of a partly closed basin about 2,000 feet deep with a sill at about 800 feet. Graded bedding, convolute structures, channels filled with shallow-water deposits, and a high percentage of displaced fauna indicate that much of the sediment was emplaced by turbidity currents and slumping. Planktonic Foraminifera indicate that late Miocene sea-surface temperatures in the Grays Harbor area were of the order of 10-15°C. A small terrestrial flora reflects a mild temperate climate. End_of_Article - Last_Page 613------------

Stratigraphy of Montesano Formation, Washington: ABSTRACT

The Montesano Formation is known to occur over about 250 square miles of Grays Harbor basin. Eight stratigraphic sections were measured along the branches of the Wishkah and Satsop Rivers, the Wynoochee River, and the Canyon River. Exposures of the Montesano Formation along the Middle Fork of the Wishkah River are designated the type section. There it is 2,500 feet thick and is composed of 1,500 feet of fine-grained sandstone, with small amounts of pebble conglomerate and mudstone, overlain by 1,000 feet of tuffaceous mudstone and sandy siltstone. Toward the east the thickness of the formation averages only 1,800 feet, and it is composed principally of fine- to medium-grained sandstone, pebbly sandstone, and conglomerate. Along the West Fork of the Satsop River, an abnorm lly thick sequence of thin-bedded to laminated, tuffaceous mudstone and very fine-grained sandstone at least 1,100 feet thick contributes to a formation thickness that may exceed 3,000 feet. Deposition took place in a sea which was transgressing eastward across a broad, east-west-trending embayment. Estimated water depths ranged from sea-level to more than 3,000 feet. The upper parts of the eastern sections apparently represent a regressive phase. Turbidite deposition in a partly closed basin was the principal cause of the abnormally thick accumulation of the thin-bedded sequence mentioned previously. Foraminiferal evidence places the Montesano Formation mainly in the upper Miocene. It is unconformable on the lower Miocene Astoria and Oligocene Lincoln Formations. A unit sometimes referred to as the Satsop Formation, and questionably considered to be Plio-Pleistocene, overlies the Montesano unconformably. End_of_Article - Last_Page 647------------