Abstract The linear Sequatchie anticline interrupts the continuity of the Appalachian Cumberland Plateau from east-Central Tennessee southward into Alabama to near the latitude of Birmingham. The anticline was breached by erosion during the late Tertiary, thereby producing Sequatchie Valley and revealing the details of its geologic structure—the anticline is thrust faulted on its northwest flank, and that thrust is now known to be part of a tectonic ramp that extends upward from the Lower Cambrian Rome Formation to flatten to the northwest into a higher detachment within the weak shale and coal beds in the Pennsylvanian deltaic sedimentary rocks. The same thrust emerges to the northwest as the Cumberland Plateau overthrust, and appears to be a mirror-image analog of the Pine Mountain fault located in the Plateau to the northeast. The purpose of this one-day field trip is to (1) provide an introduction to the Sequatchie Valley structure and the Mississippian-Pennsylvanian strata that form the crest and limbs of the anticline, and (2) gain some insight into the evolution of the topography in the southern Cumberland Plateau as the valley was exhumed during the late Tertiary. The first field trip stop is along Tennessee State Route (SR) 8 northwest of Dunlap to examine well-exposed rocks and structures along the upper detachment where it propagates along coal and shale beds in the Pennsylvanian section. The second field trip stop is up the southeast flank of the anticline along Tennessee SR-111 east of Dunlap to review the nearly continuous exposure of the Paleozoic section from the Devonian-Mississippian Chattanooga Shale to the top of the Mississippian.

Data from thousands of coalbed methane wells, conventional oil and gas wells, and five regional seismic-reflection profiles show evidence of relationships among multiple extensional events and Appalachian thrusting in the area of the Alabama Promontory and Black Warrior Basin. The oldest extensional event is of late Precambrian to early Middle Cambrian age, associated with Iapetan rifting. Along the southeastern margin of the promontory, in the northeastern part of the Birmingham graben system, a fill sequence older than the Rome Formation (Early Cambrian) is inferred. Normal faults along both sides of the promontory were active from Early Cambrian to early Middle Cambrian, as indicated by expanded hangingwall sections of the Rome and Conasauga Formations. In the Black Warrior Basin, some basement-involved normal faults were active during deposition of the Ketona and Knox carbonates (Late Cambrian–Early Ordovician). Middle Cambrian to Early Ordovician extension coincided with the inception and opening of the Rheic Ocean. Small amounts of growth occurred on some normal faults and on folds at the leading edge of the Appalachians during deposition of the Pottsville Formation (early Pennsylvanian, Morrowan). Major thin-skinned and basement-involved normal faulting occurred in the Black Warrior Basin after deposition of the preserved Pottsville section, probably during Atokan time. The extensional thin-skinned detachments are in or at the base of the Pottsville Formation and the top of the Conasauga Formation. Major Appalachian thrusting occurred after the main episode of normal faulting, perhaps during the late Pennsylvanian.

Multiple levels of frontal ramps and detachment flats accommodate tectonic shortening in contrasting deformation styles at different levels in a mechanically hetero geneous stratigraphic succession in a foreland thrust belt. The late Paleozoic Appalachian thrust belt in Alabama exhibits a balance of shortening in contrasting deformation styles at different stratigraphic levels. The regional décollement is in a weak unit (Cambrian shale) near the base of the Paleozoic succession above Precambrian crystalline basement rocks. Basement faults, now beneath the décollement, controlled the sedimentary thickness of the Cambrian shale and the location of high-amplitude frontal ramps of the regional stiff layer (Cambrian-Ordovician massive carbonate); shortening in a mushwad (ductile duplex) from thick Cambrian shale is balanced by translation of the regional stiff layer at a high-amplitude frontal ramp above a basement fault. A trailing, high-amplitude, brittle duplex of the regional stiff layer has a floor on the regional décollement and a roof that is also the floor of an upper-level, lower-amplitude, brittle duplex. The roof of the upper-level brittle duplex is a diffuse ductile detachment below an upper-level mushwad, with which parts of the brittle duplex are imbricated. The basal detachment of the upper-level mushwad changes along strike into a frontal ramp at a location coincident with a sedimentary facies change in the weak shale unit that hosts the mushwad. The roof of the upper-level mushwad is a brittle massive sandstone. Shortening on the regional décollement is balanced successively upward through contrasting tectonic styles in successive mechanically contrasting stratigraphic units.

New outcrops created during the 1983 draining of Watauga Lake within the Mountain City window exposed the Little Pond Mountain thrust zone, marked by more than 200 m of Max Meadows-type carbonate breccia. The breccias are derived from the lower Rome Formation of the hanging wall, which is thrust 12 km over younger Rome beds. The upper boundary of the fault zone is gradational, beginning with intact shales and dolostones that become progressively disaggregated by boudinage and disharmonie folding, grading into a thick zone of polymict breccia in contact with Rome shales in the footwall. The brecciation is thrust related and tied to a particular stratigraphie horizon. Extreme competency contrast between brittlely deformed dolostones and shales, and interlayered, plastically deformed calcitic laminites is inconsistent with the current mineralogy and suggests the presence of weak evaporite-rich layers during deformation. Within the fault zone, these weak beds grade into the breccia matrix. Boudinaged dolostones and shales form clasts in a breccia mixed by mesoscopic isoclinal folding. Raindrop prints, ubiquitous mudcracks, and evaporite crystal molds in the lower Rome Formation are consistent with evaporative depositional environments. The breccias also exhibit features of evaporite solution-collapse breccias, including sedimentary cavity fillings. Pétrographie evidence for vanished evaporites includes anhydrite inclusions, evaporite crystal molds, and chert nodules pseudomorphous after anhydrite. A sparry calcite mosaic that apparently replaced evaporite laminites also forms the breccia matrix. The evaporite hypothesis is supported by interbedded dolostone and anhydrite discovered in the subsurface at the base of the Rome. The Watauga Lake breccias are postulated to be the result of décollement thrusting within a dolostone-anhydrite sequence at the base of the Rome Formation, producing a polymict evaporite-matrix breccia, which after deformation, underwent local solution collapse and widespread replacement of anhydrite by calcite. The Max Meadows breccias have long been considered unique, but a review of published work shows that these rocks and occurrences at Watauga Lake are identical in many ways to Rauhwacken (cornieules) of Europe and similar carbonate breccias in Nevada, the northern U.S. and Canadian Rockies, Ireland, and southern England, all of which have been interpreted as deformed carbonate-evaporite sequences. There are also similarities to carbonate breccias in Nova Scotia, northern Michigan, and the foreland of the Canadian Rockies that are purely the result of evaporite dissolution. These comparisons show that, in a given occurrence, thick carbonate breccias with similar diagenetic histories may originate from either décollement thrusting, evaporite dissolution, or a combination of the two processes.

Controversy is common concerning the sequence of thrust fault imbrication on the scale of one or several quadrangles. Regional thrusting sequences in young orogenic belts are generally from the hinterland to the foreland. This is contrary to the previously proposed regional progression of thrusting for the southern Appalachian Valley and Ridge province. This paper uses cutoff-line maps to systematically examine some of the map patterns and cross-sectional interpretations used as evidence for the foreland-to-hinterland sequence of thrusting. Idealized examples of cutoff-line maps and cross-sectional patterns for both truncated structures and stair-stepped structures can be compared with observed map patterns and previously proposed cross-sectional interpretations. This provides critical evidence for interpreting the map data. Additional critical observations can be made as to the extent that faults may be folded by underlying structures, rather than truncating them. Overall, the cutoff-line approach and the folded fault approach document that the truncated folds expected in map-pattern for a foreland-to-hinterland thrust sequence do not occur in the east Tennessee area. Folded faults and westward-younging cutoff-line patterns indicate that later faults were in front of, and beneath, earlier ones in a hinterland-to-foreland sequence.

A late Paleozoic fault duplex forms a structural culmination in the Blue Ridge-Piedmont thrust plate near Cartersville, Georgia. The duplex contains at least three major lens-shaped horses, stacked vertically and bounded by faults that branch from the sole of the Blue Ridge-Piedmont plate. The duplex telescopes older Paleozoic structures and metamorphic fabrics that are related to Taconic thrusting of the Blue Ridge over the North American shelf. The duplex, embedded in the sole of the Blue Ridge-Piedmont plate, may have been detached from the area of the Pine Mountain window in the central Georgia Piedmont, and horizontally displaced 130 km during formation of the Valley and Ridge fold and thrust belt.

Within the southern and central Appalachians, Grenville-age basement rocks are found in major massifs in the Blue Ridge and Sauratown Mountains anticlinoria and in the vicinity of the Grandfather Mountain window. These massifs are, respectively, Pedlar and Lovingston Massifs in the Blue Ridge anticlinorium, Sauras Massif in the Sauratown Mountains anticlinorium, and Watauga, Globe, and Elk River Massifs near the Grandfather Mountain window. In central Virginia the Lovingston Massif is juxtaposed against the Pedlar Massif, and in northwestern North Carolina-southwestern Virginia, the Elk River Massif is thrust over the Globe and Watauga Massifs, all along faults of the Fries fault system, which includes the Rockfish Valley, Fork Ridge, Devil’s Fork, and Linville Falls faults, as well as the Fries fault per se . The Pedlar Massif is a deeper granulite facies country-rock terrane intruded by charnockite plutonic suites. The Lovingston Massif primarily is a shallower granulite/amphibolite facies terrane intruded by biotite dioritoid plutonic suites containing bodies of charnockite. Country rocks of the Watauga Massif were subjected to metamorphic conditions similar to those of the Lovingston Massif, but were intruded by a plutonic suite of biotite dioritoid, biotite granitoid, and granitoid. The Elk River, Globe, and Sauras Massifs all are terranes metamorphosed to amphibolite facies and intruded by granitoid/dioritoid suites containing some porphyritic biotite dioritoid phases. A suite of late Precambrian (post-Grenville) peralkaline granitoid plutons intruded all of the massifs except the Pedlar. These plutons presumably are related to upper Precambrian volcanic rocks that were associated with a rifting environment and that were later metamorphosed and deformed along with overlying sedimentary rocks to form part of the Appalachian orogenic belt.