Record Details
The oligocene and miocene geology of the Tillamook embayment Tillamook County, northwest Oregon
ScholarsArchive at Oregon State University
| Field | Value |
|---|---|
| Title | The oligocene and miocene geology of the Tillamook embayment Tillamook County, northwest Oregon |
| Names |
Parker, Michael J. (Michael John), 1958-
(creator) Niem, Alan (advisor) |
| Date Issued | 1990-03-27 (iso8601) |
| Note | Graduation date: 1991 |
| Abstract | Eleven sedimentary and volcanic rock units are mapped and described in the thesis area, and chronicle the dynamic geologic history of the Tillamook embayment from the Oligocene through the middle Mlocene. The oldest unit is the Zemorrian to early Saucesian Smuggler Cove formation, a bathyal tuffaceous mudstone with some thin- to thick-bedded tuff layers deposited on the middle to upper continental slope during a period of explosive silicic volcanism in the Western Cascade arc. Uppermost Smuggler Cove strata are coarser grained, grading upward to arkosic turbidite sandstone and mudstone and thick bloturbated silty sandstone deposited on the outer shelf during marine regression. This regression heralded the progradation of the overlying shallow-marine Bewley Creek formation (informal) depositional system. The Bewley Creek formation (informal) is proposed in this study for a sequence of pumiceous, volcaniclastic-rich lower Miocene feldspathic litharenites and lithic arkoses deposited during the Pillarlan-stage near the mouth of an ancestral Columbia River. The unit grades from bioturbated silty sandstone to fine-grained hummocky cross-stratified and coarser grained channelized sandstones deposited within, or peripheral to a wave-dominated delta or ebb tidal-delta channel complex. Progradatlon of the Bewley Creek formation may have been caused, in part, by increased volcaniclastic sedimentation attending a pulse of explosive volcanism in the adjacent Western Cascade arc. Reduced volcanic activity, possibly coupled with basin subsidence or eustatic sea level rise, resulted in deposition of mudstones of the Sutton Creek member (informal; proposed) of the Nye Mudstone. The Saucesian Sutton Creek member consists of bathyal, laminated, carbonaceous, and moderately tuffaceous mudstone deposited in an upper continental slope basin. The upper part of the unit contains common lithic to arkosic turbidite sandstone interbeds within nested channel-fill deposits. These strata represent a channelized shelf-slope break environment adjacent to the shallow-marine Angora Peak member of the Astoria Formation depositlonal system. Subsequent marine regression resulted in progradation of the Pillarian- to Newportian-stage arkosic-micaceous sandstone-rich Angora Peak member into the Tillamook embayment. Grainsize analysis, sandstone petrography, scanning electron microscopy, and heavy mineral analyses suggest these lower to middle Miocene mollusk-bearing, fine- to medium-grained sandstones were predominantly deposited near the mouth of an ancestral Columbia River. They accumulated on a high-energy Inner shelf within or down drift of a wave-dominated delta or ebb-tidal delta complex, evidenced by paleocurrent analyses, hummocky cross-stratification and trough cross-stratified submarine channel-fill sequences. The Angora Peak member disconformably overlies Zemorrian mudstones of the Smuggler Cove formation at Cape Kiwanda suggesting local uplift and erosion in that area, followed by Newportian stage transgression in the Tillamook embayment. Exotic cobbles and boulders of two mica granite and sedimentary quartzite at Cape Kiwanda were probably derived from the Idaho Batholith and Precambrian sandstone terrains in Montana, transported via an ancestral Columbia River and longshore current to the shelf possibly bound within tree root bundles. The Netarts Bay member (informal) of the Astoria Formation Is proposed In this study for a late Sauceslan package of fine-grained to pebbly amalgamated and interbedded turbidite, grainflow, and fluldized flow friable thick-bedded lithic arkoses. These massive sandstones contain large penecontemporaneously emplaced channel wall-blocks and naller slltstone rip-ups. These lower to middle Miocene strata were deposited in a submarine canyon head and channel complex offshore of the shallow-marine Angora Peak member depositlonal system. Netarts Bay strata cut Into the underlying Angora Peak shelf strata, and cut and Interfinger with bathyal slope mudstones of the Cannon Beach member of the Astoria Formation. The overlying lower Cannon Beach member Is composed of laminated bathyal mudstones with rare turbidite sandstone interbeds deposited in a coarse clastic-starved slope environment. Upper Cannon Beach member strata In the Tillamook area are characterized by micaceous arkosic and lithic arkosic turbidite sandstones that underlie and occur within nested channel-fill sequences. Bloturbated carbonaceous cross-bedded sandstone In the upper Cannon Beach member records shallowing of the Tillamook embayment to a channelized upper slope to shelf environment. The Tillamook embayment was uplifted and dissected prior to the arrival of six to ten Intracanyon subaerial and submarine lava flows of the Columbia River Basalt Group. These middle Miocene flows, delineatedon the basis of geochemical composition and magnetic polarity, Include (in stratigraphic order) the Grouse Creek (R2 low MgO-low T102), Winterwater (N2 low MgO-low Ti02), and Sentinel Bluffs (high MgO) units of the Grande Ronde Basalt, and the Ginkgo unit of the Frenchman Springs member of the Wanapum Basalt. Orientation of Grande Ponde Basalt foreset-bedded pillow palagonite complexes and lava delta sequences indicate that these Columbia River Basalt units flowed westward into the Tillainook embayment, possibly through a saddle in the ancestral Oregon Coast Range. Marine transgression and deposition of hummocky cross-stratified arkosic marine strata of the Sandstone of Whale Cove followed emplacement of the last Grande Ronde flows. This was succeeded by a regression, as Indicated by the overlying subaerial plagioclase-phyric Ginkgo Unit flow of the Frenchman Springs Basalt. Locally, Winterwater and Sentinel Bluffs unit basalt occur as brecciated peperitic sills and dikes. These were emplaced through the process of "auto-invasion" when dense lava injected downward Into semi-lithifled Tertiary strata under the influence of both a pressure head augmented by flashing steam, and steam blasting. The thesis area is crossed by a complex network of high-angle northwest- and northeast-tending normal and reverse faults, and both low and high-angle east-trending reverse and thrust faults. These faults may have developed through a north-south compressional tectonic regime, a dextral shear couple, or a combination of these two tectonic regimes. Many faults cut Columbia River Basalt units and are thus middle Miocene or younger in age. Tertiary strata including the Columbia River Basalts are also folded within a broad westward-plunging syncline which suggests a middle Miocene or younger compressional event. Source rock analyses indicate that the mudstones of the Cannon Beach member, Sutton Creek member, and Smuggler Cove formation contain type III kerogen capable of generating natural gas only. Although these rocks have thermally innature vitrinite reflectance values, they contain sufficiently high total organic carbon content to be considered potential lean source rocks. Arkosic sandstones of the Angora Peak and Netarts Bay members have fair to good reservoir rock characteristics, and may represent reservoirs offshore for matured hydrocarbons generated from deeply buried source rocks. |
| Genre | Thesis |
| Topic | Geology, Stratigraphic -- Oligocene |
| Identifier | http://hdl.handle.net/1957/9478 |
