ABSTRACT This one-day field trip will explore the geomorphology, landslide mapping, geochronology, tectonics, meteorology, and geoengineering related to the Blue Ridge Escarpment (BRE), North Carolina, USA. Our aim is to show why it has persisted in the landscape and how it influences landslide frequency and the lives of the western North Carolina people. Some of the work we highlight has been published and some we present for the first time. Landslides pose a frequent geologic hazard to the people of western North Carolina, and they cause losses of road access, property, or, in the worst scenarios, human lives. We will also discuss landslide disaster response and mitigation efforts that required the collaboration of state and local emergency managers with other local, state, and federal agencies and the public. As we traverse the rugged terrain along the BRE in Polk and Rutherford counties, we will examine rockfalls, rockslides, debris flows, and debris slides occurring in late Proterozoic to early Paleozoic metasedimentary and meta-igneous rocks southeast of the Brevard fault zone. Our focus will be steep-walled, topographic reentrants where streams exploit brittle, post-orogenic bedrock structures, incise into the BRE, and produce landforms prone to debris flows and other types of mass wasting, often triggered by extreme rainfall events. The research we present on these extreme historical storms will help illustrate the scope and magnitude of the BRE’s influence on meteorological and hydrological events that lead to landslides and flooding. In addition to ongoing countywide landslide hazard mapping, a complementary research objective is to better understand the influence brittle cross-structures and earlier ductile bedrock structures have on rock slope failures and debris flows in the North Pacolet River valley and Hickory Nut Gorge, two major structurally controlled topographic lineaments.

Abstract Detailed geologic mapping and new SHRIMP (sensitive high-resolution ion microprobe) U-Pb zircon, Ar/Ar, Lu-Hf, 14 C, luminescence (optically stimulated), thermochronology (fission-track), and palynology reveal the complex Mesoproterozoic to Quaternary geology along the ~350 km length of the Blue Ridge Parkway in Virginia. Traversing the boundary of the central and southern Appalachians, rocks along the parkway showcase the transition from the para-autochthonous Blue Ridge anticlinorium of northern and central Virginia to the allochthonous eastern Blue Ridge in southern Virginia. From mile post (MP) 0 near Waynesboro, Virginia, to ~MP 124 at Roanoke, the parkway crosses the unconformable to faulted boundary between Mesoproterozoic basement in the core of the Blue Ridge anticlinorium and Neoproterozoic to Cambrian metasedimentary and metavolcanic cover rocks on the western limb of the structure. Mesoproterozoic basement rocks comprise two groups based on SHRIMP U-Pb zircon geochronology: Group I rocks (1.2-1.14 Ga) are strongly foliated orthogneisses, and Group II rocks (1.08-1.00 Ga) are granitoids that mostly lack obvious Mesoproterozoic deformational features. Neoproterozoic to Cambrian cover rocks on the west limb of the anticlinorium include the Swift Run and Catoctin Formations, and constituent formations of the Chilhowee Group. These rocks unconformably overlie basement, or abut basement along steep reverse faults. Rocks of the Chilhowee Group are juxtaposed against Cambrian rocks of the Valley and Ridge province along southeast- and northwest-dipping, high-angle reverse faults. South of the James River (MP 64), Chilhowee Group and basement rocks occupy the hanging wall of the nearly flat-lying Blue Ridge thrust fault and associated splays. South of the Red Valley high-strain zone (MP 144.5), the parkway crosses into the wholly allochthonous eastern Blue Ridge, comprising metasedimentary and meta-igneous rocks assigned to the Wills Ridge, Ashe, and Alligator Back Formations. These rocks are bound by numerous faults, including the Rock Castle Creek fault that separates Ashe Formation rocks from Alligator Back Formation rocks in the core of the Ararat River synclinorium. The lack of unequivocal paleontologic or geochronologic ages for any of these rock sequences, combined with fundamental and conflicting differences in tectonogenetic models, compound the problem of regional correlation with Blue Ridge cover rocks to the north. The geologic transition from the central to southern Appalachians is also marked by a profound change in landscape and surficial deposits. In central Virginia, the Blue Ridge consists of narrow ridges that are held up by resistant but contrasting basement and cover lithologies. These ridges have shed eroded material from their crests to the base of the mountain fronts in the form of talus slopes, debris flows, and alluvial-colluvial fans for perhaps 10 m.y. South of Roanoke, however, ridges transition into a broad hilly plateau, flanked on the east by the Blue Ridge escarpment and the eastern Continental Divide. Here, deposits of rounded pebbles, cobbles, and boulders preserve remnants of ancestral west-flowing drainage systems. Both bedrock and surficial geologic processes provide an array of economic deposits along the length of the Blue Ridge Parkway corridor in Virginia, including base and precious metals and industrial minerals. However, common stone was the most important commodity for creating the Blue Ridge Parkway, which yielded building stone for overlooks and tunnels, or crushed stone for road base and pavement.

The three field guides in this volume explore facets of the geology of western North Carolina, USA. The trip to Buck Creek and Chunky Gal Mountain examines high-temperature mafic and ultramafic rocks interpreted to be part of a dismembered ophiolite thrust onto Laurentian crust during the Taconic orogeny. The second trip traverses rugged terrain along the Blue Ridge Escarpment from near the South Carolina state line to Hickory Nut Gorge just southeast of Asheville to examine rockfalls, rockslides, debris flows, and debris slides triggered by extreme rainfall events. The trip to Sparta, North Carolina, highlights the results of interdisciplinary studies among collaborators following the 2020 M w 5.1 earthquake.

Abstract Seismic reflections interpreted to be top Oligocene, top Wilcox (approximately base middle Eocene), top Cretaceous, top Jurassic, and basement were mapped across portions of the Green Canyon, Keathley Canyon, Walker Ridge, Lund, Sigsbee Escarpment, Amery Terrace, and Lund South OCS areas of the central northern Gulf of Mexico (Fig. 1). 3D Pre-stack depth migrated data were used for mapping the areas covered by allochthonous salt. 2D Pre-stack time migrated data were used for mapping the area on the abyssal plain beyond the Sigsbee Escarpment. These data cover approximately 50,000 km 2 (19,500 miles 2). Well control was obtained from data available through the Minerals Management Service. Figure 1. Location map. Black line encloses the area of data coverage. Dashed line marks the transition from 3D prestack depth migrated (PSDM) data to the west and north to 2D pre-stack time migrated (PSTM) data to the east. Numbered segments refer to figures with those numbers. Abbreviations for deep-water OCS areas: AC–Alaminos Canyon; AM–Amery Terrace; AT–Atwater Valley; EB–East Breaks; GC–Green Canyon; GB–Garden Banks; KC–Keathley Canyon; L–Lund; LS–Lund South; MC–Mississippi Canyon; SE–Sigsbee Escarpment; WR–Walker Ridge. Structure maps on the top Oligocene, top Wilcox, top Cretaceous, and basement formed the regional surfaces between which isopach/isochron maps were created to analyze depositional patterns. As might be expected, basement structure displayed the greatest relief and complexity. Outboard from the allochthonous salt of the Sigsbee Escarpment, half-graben structures indicative of rift basin topography were clearly imaged (Fig. 2). Elsewhere on the abyssal plain isolated, sharp-peaked, elevated basement features were observed between more numerous gently sloped highs. These basement structures typically had reflection terminations against their margins or flanks and continuous reflections draping them. Figure 2. 2D Pre-stack time migrated line showing rift basin structure in the basement, Wilcox strata down lapping onto the Cretaceous and thinning to the east, and Oligocene strata down lapping onto the Wilcox and thinning to the north. The vertical scale is in seconds of two-way time (TWT). The horizontal scale is in feet (100,000 feet ~ 18.94 miles or 30.55 kilometers). Abbreviations for horizons: Olig=Oligocene (orange); Wx=Wilcox (blue); K=Cretaceous (green); J=Jurassic (pink); and Bsmt=basement (yellow). The top Cretaceous and top Wilcox surfaces show broad regional similarities and show less structural complexity than the basement. Outboard of the Sigsbee Escarpment, both surfaces are broadly lobate and have relatively gentle inclinations which rise to the east. The main observable differences between the two are: (A) the Cretaceous surface has several isolated high points reflecting underlying basement structures and (B) the Wilcox surface has a more lobate/interdigitate contour character. The top Oligocene surface is less lobate in appearance than either the Cretaceous or Wilcox surface and rises to the southeast (Fig. 3). Figure 3. Time structure map on the top Oligocene. The contour interval is 50 milliseconds. Isochron maps between the four structural surfaces reflect the underlying structure and depositional trends of the interval. Thus the basement to Cretaceous isochron shows thick Jurassic infill, Cretaceous drape in the grabens (Fig. 2), and thin to no cover over highs in the rifted basement topography. The Cretaceous to Wilcox isochron has a broad lobate form that thins gently from west to east. A very subtle down-lapping pattern is visible within the Wilcox interval on Figure 2 . Deviations from this pattern occur primarily where basement structures produce isolated thins. The Wilcox to Oligocene interval shows a regional gradient of north to south thickening and only a slight influence from deeper structure. Down-lapping and thinning to the north strongly suggest a southerly source for the Oligocene interval. Beneath the allochthonous salt of the Sigsbee Escarpment, all surfaces deepen northward and show much greater local variability. Basement is only occasionally visible as it generally lies below the fifteen kilometer limit of the available PSDM data. The deepest area mapped is in Green Canyon where the top Oligocene approaches twelve kilometers depth, the top Wilcox approaches thirteen kilometers, and the top Cretaceous almost fourteen and one half kilometers. These surfaces shallow to less than eight kilometers deep on the abyssal plain. Three coincident lows roughly oriented north-south suggest preferred sediment pathways and possibly areas of thicker original autothonous salt. A change on the structure and isopach maps from smooth broadly spaced contours on the abyssal plain to highly variable tightly spaced contours suggests the location for the original limits of salt deposition in this area. This location often lies close to but not exactly in line with the present day Sigsbee Escarpment (Fig. 1). Of key interest to hydrocarbon explorationists are any factors that would effect Wilcox deposition. We have observed three factors that influence the deposition and thickness of Wilcox age strata in this area: Pre-existing basement highs have caused the Wilcox to be thin or absent around those structures. Although basement topography is mostly smoothed over by the end of the Cretaceous, a few large structures still influenced deposition in the Wilcox on the abyssal plain beyond the Sigsbee Escarpment. Salt nappes and salt pillows have caused thinning of Wilcox strata over those structures. Our interpretation indicates multiple kilometer thick salt nappes extruded beyond the limits of the original salt basin during the Cretaceous (Figs. 4 and 5). Inflated salt pillows associated with the nappes lay along the boundary of the salt basin. Though now deflated, the presence of these salt pillows and other salt pillows updip are recorded by the depositional thinning of Wilcox strata above them. These allochthonous bodies provided the core structure over which Wilcox and Miocene reservoirs are folded or draped at Chinook, Atlantis, Das Bump, and other important deepwater discoveries. The location of allochthonous salt at the onset of Wilcox deposition is apparently coincident with the pronounced increase in northerly dips of the Mesozoic and Paleogene strata. This relationship is consistent with originally thick autochthonous salt above the deepest mapped basement. Sites of continued salt withdrawal from the autochthonous level into growing salt structures directly affected Wilcox sediment thickness. Such sites would have been primary candidates for the location of Wilcox sediment fairways. Identification and elimination of salt feeders would help in refining/defining these pathways. Figure 4. 3D Pre-stack depth migrated line showing a Cretaceous age salt nappe and its associated deformation front. Both features lie just basinward of the modern Sigsbee Escarpment. Thinned Wilcox and Oligocene strata show where a now evacuated salt pillow once existed. The vertical and horizontal scales are in kilometers. Abbreviations for horizons: Olig=Oligocene (orange); Wx=Wilcox (blue); K=Cretaceous (green); J=Jurassic (pink); and Bsmt=basement (yellow). Figure 5. 3D Pre-stack depth migrated line showing a second Cretaceous age salt nappe. This one lies about thirty kilometers shoreward of the Sigsbee Escarpment. Thinned Wilcox strata and an Oligocene turtle structure show where a now evacuated salt pillow once existed. The vertical and horizontal scales are in kilometers. Abbreviations for horizons: Olig=Oligocene (orange); Wx=Wilcox (blue); K=Cretaceous (green); J=Jurassic (pink); and Bsmt=basement (yellow). Deposition of the Wilcox strata can be broadly divided into two paleogeographic domains: (A) a relatively complex north-westerly region characterized by pre-existing, elevated sea-floor, salt-cored structures and sites of contemporaneous salt evacuation, and (B) a relatively simple south-easterly region characterized by a near flat and smooth sea-floor rarely punctuated by unburied basement structures. The transition between these two regions should mark changes in Wilcox depositional styles. In the more complex topographic region, Wilcox depositional events were forced to interact with relatively rapid changing sea-floor dips. Whereas in the more simple region to the southeast, a much more unconfined sea-floor presented limited impediment to widespread expansion of depositional events exiting the more complex region to the north-west. Drilling of Wilcox strata to-date has been mainly in the simpler south-easterly region and in the transition zone to the more complex Wilcox geometries towards the north-west. Figure 4 shows an example of one salt nappe and its contractional deformation front that lies in close proximity but basinward of the Sigsbee Escarpment. Thrust relationships suggest that the nappe continued to move/inflate until the end of the Cretaceous. An inflated salt pillow associated with the nappe is present through the Oligocene but then deflates during the Miocene. This interpretation is supported by the thin but depressed Wilcox and Oligocene section behind the nappe today. We predict that the edge of the salt basin lies behind the nappe, below where the Wilcox and Oligocene intervals begin dipping to the north. Figure 5 shows another example of a salt nappe that lies in about thirty kilometers inside of the Sigsbee Escarpment. This nappe does not have a deformational front associated with it. But an inflated salt pillow is associated with this nappe as in Figure 4 . Similar to Figure 4 , the interpretation is supported by a thin but depressed Wilcox section behind the nappe. In contrast, evacuation of the pillow begins in the Oligocene, as evidenced by the Oligocene age turtle structure. Evacuation continues into the Miocene until the pillow is completely deflated. The nappe remnant is all that remains of this salt body. Unique to these two examples, but possibly typical of most salt pillows around the edge of the salt basin, loading has forced salt backwards (updip) into the salt basin. In Figure 4 , the reversal of salt movement is about ten kilometers. In Figure 5 , the reversal of salt movement may be twenty to twenty-five kilometers.