Mid-Tertiary paleoseismites: Syndepositional features and section restoration used to indicate paleoseismicity, Atlantic Coastal Plain, South Carolina and Georgia

Abstract

Synclinal troughs that served as depositional channels adjacent to small reverse faults are examples of paleoseismites. Section restoration on one syncline demonstrates progressive fault growth, during three seismic events with 1.5 m of relative uplift. Normal faults, which are bent or offset, preceded reverse faults. Faults are truncated by unconformities progressively upward through the exposed stratigraphic section. One Ophiomorpha nodosa burrow cuts a reverse fault, indicating that faulting occurred within a few meters of the ground surface in a marine environment. Near-surface, small-scale reverse faults on the flanks of the second syncline accompanied folding, and record two events with ~3 m of relative uplift. At a third locality, 21 near-vertical clastic dikes are subparallel to nearby joints and small normal faults. They are 2 to 10 cm wide, 1 to 2 m long, 1 to 2 m high, relatively planar with parallel walls and sharp contacts, and filled with medium-to coarsegrained quartz sand. The dikes cut perpendicular to bedding across sandy clay beds containing Ophiomorpha nodosa burrows indicating their formation within a few meters of the ground suface in a marine environment. Collectively, extensional structures here are interpreted to have developed across a broad fold above the Pen Branch fault.The stratigraphic and geographic association of these features suggests that all are secondary structural features, interpreted herein as paleoseismites, that accompanied major paleo-earthquakes on nearby large reactivated faults like the Pen Branch fault during late Eocene to late Miocene tectonism.

Relationship between observed upper mantle structures and recent tectonic activity across the Southeastern United States

“…This region includes the devastating 1886 Charleston, SC earthquake. Studies indicate that the faults associated with this seismicity may also control the regional uplifting of coastal sediments [ Rhea , ; Marple and Talwani , ; Bartholomew et al ., ]. Farther north, the Central Virginia Seismic Zone (CVSZ, Figure ) in north‐central Virginia is characterized by a cluster of earthquakes.…”

“…The SEUS is the product of multiple episodes of tectonic accretion and rifting that have taken place since the Late Proterozoic (>1.0 Ga) [ Cook et al ., ; Thomas , ; Hatcher , ], but it has not been near a plate boundary for over 150 Myr (Figure ). Today, the SEUS is located over 1700 km away from the closest plate boundary, but it still comprises multiple zones of active seismicity, including a number of destructive earthquakes such as the 1886 Charleston, South Carolina (SC) earthquake ( M w = 7.0) [ Johnston , ; Bartholomew et al ., ] (Figure ) and the 2011 Mineral, Virginia (VA) earthquake ( M w = 5.8) [ Wolin et al ., ]. In addition to significant intraplate seismicity, various geomorphological observations point to ongoing Cenozoic epeirogeny, including evidence for recent uplift of the ancient Appalachian orogen (Figure ) [ Spotila et al ., ; Gallen et al ., ; Liu , ], northwestward migration of the Blue Ridge [ Willett et al ., ] and the presence of recently uplifted coastal arches and scarps [ Winker and Howard , ; Rowley et al ., ].…”

Sand injectites at the base of the Coconino Sandstone, Grand Canyon, Arizona (USA)

“…Clastic dikes and injectites have many of the same characteristics as the Grand Canyon cracks: 1) they are commonly massive and have no internal structures (Peterson, 1968;Hiscott, 1979), but 2) some can have flow structures (or banding) within them (Diller, 1889;Jenkins, 1925a;Harms, 1965;Peterson, 1968;Scott et al, 2009), 3) they are most commonly composed of fine to medium grained sand (Harms, 1965;Hiscott, 1979;Jolly and Lonergan, 2002;Obermeier et al, 2002), 4) they are often intimately associated with faulting and tectonic activity (Harms, 1965;Bartholomew et al, 2002;Jolly and Lonergan, 2002), 5) they can be closely spaced (Diller, 1889;Newsom, 1903;Waterston, 1950;Harms, 1965), 6) they can sometimes have zones of shearing within (Harms, 1965;Petit and Laville, 1987;Boehm and Moore, 2002) and 7) they are preferentially oriented (discussed in the next section). The cause of the slickenside-like features found at Bright Angel Trail and Jumpup Spring is uncertain.…”

“…The Bright Angel Fault (and its associated faults) is an appropriate tectonic setting for such features to form. Clastic dikes are well known features in various tectonic settings (Harms, 1965;Bartholomew et al, 2002). The Hermit and Coconino are an appropriate depositional setting for clastic dikes to form.…”

Initial Laramide tectonism recorded by Upper Cretaceous paleoseismites in the northern Bighorn Basin, USA: Field indicators of an applied end load stress

“…Soft-sediment deformational structures form in response to natural processes such as rapid sedimentation and paleoseismicity (Obermeier, 1996;Audemard and Michetti, 2011;Owen and Moretti, 2011). Paleoseismites, defined as pre-Neogene soft-sediment deformational structures associated with paleoseismicity, record syndepositional tectonism prior to lithification and the onset of major orogenic events (Winslow, 1983;Bartholomew et al, 2002;Stewart et al, 2002;Bartholomew and Whitaker, 2010). Upper Cretaceous through Eocene strata in the northern Bighorn Basin (Wyoming, USA) contain paleoseismites that are interpreted to record Laramide deformation in the region (Bartholomew et al, 2008;Stewart et al, 2008;Jackson et al, 2016).…”