IMPOSTER UNCONFORMITY: PART OF THE MISSING AND INCOMPLETE RECORD IN ROCKS

Transcription

1 IMOTE UNONFOMITY: AT OF THE MIING AND INOMLETE EOD IN OK Habte Giorgis hurnet The University of Tennessee at hattanooga How much credence may be placed on an incomplete record that is bounded by missing records? Hawaii International onference on science, 11:40-12:05 AM, January 16, 2004 Menz High Yearbook: Missing & Incomplete ecord Yearbook 1973 Menz High 1972 Menz High 1971 Menz High 1960 Menz High 1959 Menz High 1958 Menz High 1957 Menz High unconformity Each yearbook is an incomplete record, for it does not contain all school activities of a year. Yearbooks 1961 through 1970 are missing = Unconformity. Is the record for missing because yearbooks were not prepared for these years, or because they were destroyed? Is there necessary and sufficient data to make valid inferences. For example, can we determine the ratio of black/white soccer players, if that were what a study involved McBride High at Gatwick 1989 McBride High at Gatwick 1988 McBride High at Gatwick 1987 McBride High at Gatwick 1986 McBride High at Gatwick 1960 McBride High upon erth 1959 McBride High upon erth At Gatwick Imposter Unconformity In 1985, pursuant to a decision to relocate McBride High, the mayor of McBride ity lifted old yearbooks, threw away younger ones (1961 through1984) from McBride High upon erth, and placed only the 1959 & 1960 yearbooks at Gatwick. In subsequent years, yearbooks from McBride at Gatwick were placed on top of those as they were published. 1

2 ummary 1) The process represented by an unconformity might be much shorter in duration than implied by the ages of rocks on either side of an unconformity as in the case of McBride High. uch an unconformity is called an Imposter Unconformity 2) Discussion on imposter unconformity alerts us to pay attention to geologic processes at a locality, rather than to emphasize on the ages of rocks on either side of the unconformity. 3) Good examples, such as using yearbooks, enable the student to grasp the difficult idea of unconformity. 4) Incomplete or missing record when viewed globally may contain necessary and sufficient data for a particular study. Foreword to subsequent Diagrams ome Geologists suggest that the Ocoee eries of the Tennessee Blue idge are or younger. Many Others think that the Ocoee are recambrian. The idea of an imposter unconformity was proposed to explain the unconformity between a possible Ocoee and recambrian rocks could have been of short duration, and within the (see subsequent diagrams. roposed Imposter unconformity surfaces in the Grand anyon and Death Valley are shown on photographs and diagrams. Tectonostratigraphic sketch of southeastern Tennessee ennsylvanian Mississippian Devonian Ordovician Gizzard Group ennington Fm. Mississippian. arbonates hattanooga hale ockwood Fm. - linch s. equatchie Fm. hickamauga Fm- evier h. Ocoee eries? Alleghanian Acadian Taconian Orthodox View Walden reek Group Great moky Group Ocoee eries andsuck Formation Wilhite Formation hields Formation Unrag and others, 1993, 1999 L. ambrian fossils ambrian recambrian Knox Gp. onasauga Gp. ome Fm. hady Dol. hilhowee G. Ocoee series? Basement assive margin Grenvillian nowbird Group The surface between the recambrian basement and the Ocoee eries might be an imposter unconformity. 2

3 Younger than Lower ambrian Lower ambrian Western Blue idge Ocoee eries(orange) bounded by Lower ambrian rocks (Green), and underlain by ambrian to Lower Ordovician rocks (blue). Are the Ocoee recambrian? Or are they and younger? Klippe? Western Blue idge roponents of recambrian age for the Ocoee suggest: 1) An unconformity separates the Ocoee from the basement. 2) The contact between the Ocoee and ambrian strata is conformable. 3) ubsequent deformation and erosion resulted in what is now. ambrian-l.ordovician L. ambrian recambrian Western Blue idge roponents of or younger age for Ocoee eries suggest: 1) L.ambrian directly lies on recambrian 2) Uplift and erosion, including of the basement Denudational 3) and/or pull apart faulting and Imposter unconformity deposition ubsequent deformation and erosion resulted in current disposition. 3

4 atching Angular Unconformity: iccar oint, cotland Grand anyon. View from Tonto lateau,arizona across the olorado to Utah. Mississippian Dis. ambrian Devonian Old ed Non. Angular Grand canyon upergroup ,250 MA Imposter Turbidites Hutton s impact: hattered a belief that Earth was created 4004 B. Unleashed human thought to ponder vast time with its immense possibilities. Death Valley, A. View of Black Mountains from anamint ange. Imposter Unconformity surface between recambrian wall rock and enozoic sediment fill. Death Valley is a ull-apart basin formed subsequent to uplift, erosion, and denudational faulting that propagated westward from pring Mountains.. Vishnu and Zoroaster. 1,400 to 2,000 MA Unconformities : Angular, Disconformity (Dis.), Nonconformity (Non.), and Imposter Unconformity Black Mountains anamint ange Death Valley F 4

5 < 30 MA Vs 600 MA Alluvial fan at the foot of Black Mountains, Badwater. UG hoto. urface across the recambrian rock and alluvial fans: Imposter Unc. UG hoto. Bahada on anamint ange, western boundary of Death Valley. ontact between recambrian and overlying young sediments is an imposter unconformity. Ocoee likely is submarine equivalent. onclusion H G F E D a. Angular unconformity B A D B b. Disconformity A 1) An imposter unconformity represents a missing record, which is much shorter in duration than the gap in age estimated from rock ages bounding the unconformity surface. T T 2) When a surface separates rock layers in obvious discordance (fault-like) on one side of an inferred depositional basin, and its continuation appears to be a nonconformity, that surface is an imposter unconformity. c. Nonconformity d. Imposter unconformity 5