File:Millbrig Bentonite in limestone succession (Upper Ordovician, 454 Ma; Warren County core, Ohio, USA) 1.jpg
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English: Bentonite from the Ordovician of Ohio, USA.
Bentonites are volcanic ash beds. Several are present in Ordovician-aged sedimentary successions in eastern America. The slightly crumbled, friable rocks seen in the middle of the picture are part of an Ordovician-aged bentonite interval in the famous Warren County core of Ohio. The rocks at top and bottom are limestones. The Ohio Geological Survey drilled a core from 1987 to 1989 in northeastern Warren County, Ohio. It was intended to be a stratigraphic reference section from the Upper Ordovician to Precambrian basement rocks. Instead of encountering igneous or metamorphic rocks below the Cambrian sedimentary cover, the core unexpectedly penetrated a thick, late Precambrian-aged sedimentary succession, which has been interpreted as a rift-basin fill. The rift fill sedimentary rocks were a new stratigraphic formation now called the Middle Run Formation. The Ordovician portions of the core include this prominent bentonite bed, the Millbrig Bentonite. It is about 1.67 feet thick in the core. The Millbrig has been traced throughout eastern America, from the Upper Mississippi Valley into the Appalachians. The source volcano was likely somewhere in what is now America's Eastern Seaboard. During the Ordovician, the Taconic Orogeny was taking place in this part of Laurentia (= the ancient North American continent). The Taconic Mountains were being uplifted by the collision of a volcanic island arc, which was itself formed by subduction. Subduction zones always produced numerous volcanoes that have explosive ash eruptions. The volume of volcanic ash in the Millbrig Bentonite horizon is enormous - considerably larger than Indonesia's Toba Volcano eruption at 74 ka. Stratigraphy: Millbrig Bentonite, near the boundary between the Lexington Limestone and the Black River Limestone, upper Middle Ordovician (sensu traditio) / lower Upper Ordovician (sensu novum), ~454 Ma Locality: 1097.13 to 1098.80 feet interval (= feet below the surface well site), Ohio Division of Geological Survey core 2627, American Aggregates Corporation limestone quarry (now flooded), just northeast of the town of Lytle, northwestern Wayne Township, northeastern Warren County, southwestern Ohio, USA Core-specific info. from: Shrake (1991) - The Middle Run Formation: a subsurface stratigraphic unit in southwestern Ohio. Ohio Journal of Science 91: 49-55. See info. at: <a href="https://en.wikipedia.org/wiki/Deicke_and_Millbrig_bentonite_layers" rel="noreferrer nofollow">en.wikipedia.org/wiki/Deicke_and_Millbrig_bentonite_layers</a> and <a href="https://en.wikipedia.org/wiki/Bentonite" rel="noreferrer nofollow">en.wikipedia.org/wiki/Bentonite</a> |
| Date | |
| Source | Own work |
| Author | James St. John |
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