Edward C. Beutner

Post–middle Miocene accretion of Franciscan rocks, northwestern California

Deformed sedimentary rocks assigned to the Franciscan assemblage in the King Range south of Cape Mendocino, northern California, are dominantly deep-water argillite and sandstone occurring as thick- to thin-bedded, locally channelized marine turbidites of arkosic to andesitic volcani-clastic composition. These rocks structurally overlie Late Cretaceous (Coniacian to Campanian) basaltic pillow flows, flow breccias, diabase sills, and arkosic sandstone turbidites that also are assignable to the Franciscan assemblage, and are well exposed along the coast at Point Delgada. Radiolarians and planktic and benthic foraminifers indicate that the oldest rocks in the King Range may be early Tertiary and that the youngest rocks are middle Miocene or younger, the youngest rocks yet reported from the Franciscan assemblage. Partly coeval Miocene (Relizian and Luisian) to Quaternary rocks, representing shallower sedimentary facies of the inner shelf to outer continental slope, occupy the Eel River basin and isolated fault-bounded slivers adjacent to the King Range on the north and east. Wide zones of penetrative shearing that may delineate a middle Miocene or younger suture separate the deep-water Franciscan rocks of the King Range from less severely deformed Neogene rocks of the Eel River basin. The King Range appears to be a displaced terrane of oceanic basement overlain by Paleogene(?) and Neogene sedimentary and igneous rocks of continental and oceanic derivation. This terrane may have been displaced north or eastward and obductively accreted to California ≤15 m.y. ago, before or perhaps during northward passage of the Pacific-Farallon-North American plate junction.

Large volume loss during cleavage formation, Hamburg sequence, Pennsylvania

Green reduction spots in red slate of the Hamburg sequence exposed near Shartlesville, Pennsylvania, have axial ratios of 1.42:1.0:0.28 on the limbs of near-isoclinal folds and 1.0:0.79:0.41 in fold hinge zones. Conodont cusps and denticles within the reduction spots have been brittlely pulled apart and give independent measures of extension in various directions. Comparison of conodont extensions with reduction spot shapes on limbs and hinges indicates that sedimentary compaction of 44% preceded the tectonic strain associated with cleavage formation. This strain, having identical maximum extensions but greater shortening in fold hinges as compared to limbs, was characterized by 41% extension in X, no change in Y, 50% to 59% shortening in Z, and 29% to 42% tectonic volume loss. The general lack of directed overgrowths on grains reflects the large volume loss and contrasts with other slates, where deformation was an almost constant volume process and extension in X compensated for shortening in Z.

Catastrophic emplacement of the Heart Mountain block slide, Wyoming and Montana, USA

The mechanism that allowed many tens of km of movement of the enormous block slide floored by the rootless Heart Mountain detachment fault in NW Wyoming has long been a puzzle. Carbonat-rich microbreccia that is widespread along the fault and in dikes in the upper plate contains accreted grains indistinguishable from those observed as fallout from volcanic eruption clouds (accretionary lapilli) and impact ejecta clouds and in intrusive diatremes. In these settings and also in industrial processing, accreted grains form when particles in a turbulent gaseous suspension containing limited water adhere to a nucleating grain or to each other. Elongate grains in thick microbreccia have strong but diverse shap-preferred orientations unlike those reported from other fault rocks but instead suggestive of turbulent flow, and the microbreccia contains layering and other features of sedimentary character that appear to record deposition from suspension rather than frictional processes along a fault. We suggest that frictional heating led to dissociation of carbonate rock along the fault, producing supercritical CO 2 as the suspending medium. High CO 2 pressure drastically reduced friction along the fault and allowed continuation of catastrophic movement, probably initiated by a volcanic or phreatomagmatic explosion, resulting in very large displacement on a low-dipping surface. Earlier slower sliding may have occurred but final emplacement was rapid (minutes) and spectacular.

Volcanic fluidization and the Heart Mountain detachment, Wyoming

The presence along the Heart Mountain detachment in Wyoming of microbreccia containing volcanic glass grains with primary shapes and accreted grains equivalent to accretionary and armored lapilli supports the concept that injection of volcanic gases along the fault produced fluidization. The probable source of the volcanic contribution was a fixed feeder pipe, now beneath the Crandall intrusive complex, which left a trail of intrusives akin to a hotspot trace in the moving allochthon. Volcanic gas carrying glass and fluidized microbreccia was injected in sill-like fashion along a bedding horizon near the base of the Ordovician Bighorn Dolomite, resulting in gravitative collapse and spreading, probably catastrophic, of the overlying carbonate and volcanic massif.

Northern San Andreas fault near Shelter Cove, California

The location of the San Andreas fault in the Shelter Cove area of northern California has been the subject of long-standing debate within the geological community. Although surface ruptures were reported near Shelter Cove in 1906, several subsequent workers questioned whether these ruptures represented true fault slip or shaking-related, gravity-driven deformation. This study, involving geologic and geomorphic mapping, historical research, and excavation across the 1906 rupture zone, concludes that the surface ruptures reported in 1906 were the result of strike-slip faulting, and that a significant Quaternary fault is located onshore near Shelter Cove. Geomorphic arguments suggest that the Holocene slip rate of this fault is greater than about 14 mm/yr, indicating that it plays an important role within the modern plate-boundary system. The onshore trace of the fault zone is well expressed as far north as Telegraph Hill; north of Telegraph Hill, its location is less well-constrained, but we propose that a splay of the fault may continue onshore northward for at least 9 km to the vicinity of Saddle Mountain.

BEDDING GEOMETRY IN A PENNSYLVANIAN CHANNEL SANDSTONE

Cross-bedded sandstone in the Pennsylvanian Kittanning Formation of the Allegheny Group exposed in a strip mine near Philipsburg, Pennsylvania, consists of large, inclined, sigmoid sedimentation units that overlap each other uniformly toward the west. Such features as the arrangement of sedimentation units and their internal bedding geometry suggest that the sandstone originated by deposition on the inside of a meander curve of a westwardly migrating stream channel.

Reverse Gravitative Movement on Earlier Overthrusts, Lemhi Range, Idaho

Two large gravity slide blocks moved up to 5 mi southwestward off the Lemhi Range during the latest stages of basin-range uplift. Both detachment faults coincide in part with folded thrust faults along which 1 to 3 mi of eastward displacement occurred during the Mesozoic Sevier orogeny. The detachment faults, which dip 9° to 12° southwest, are not offset by the normal fault zone at the range margin. Sliding was localized by the interception of west-dipping folded thrust surfaces by the range front normal fault zone.

Dewatering origin of cleavage in light of deformed calcite veins and clastic dikes in Martinsburg slate, Delaware Water Gap, New Jersey

Martinsburg slate near Delaware Water Gap contains two sets of precleavage calcite veins, both probably related to brittle failure during early flexural folding. Laminated veins that lie along bedding-plane faults of small displacement are locally folded, whereas veins that were originally perpendicular to bedding have boudinage structure. The latter have been used to define the strain ellipsoid (axial ratio 2.08:1:0.48) for the deformation accompanying cleavage formation. Clastic dikes are at various angles to cleavage and bedding and do not provide evidence for cleavage having formed prior to lithification. Slaty cleavage in the Martinsburg formation at this classic locality formed in lithified pelitic flysch and is unrelated to dewatering.

Slaty cleavage unrelated to tectonic dewatering: The Siamo and Michigamme slates revisited

Clastic dikes in slate in the Precambrian Michigamme and Siamo Formations of northern Michigan have been described as being parallel to slaty cleavage; this relationship was then used to support the hypothesis that the cleavage formed in response to tectonic dewatering. Careful measurement of cleavage and dike attitudes at the two previously studied localities demonstrates that the dikes are not parallel with cleavage. The dikes and other soft-sediment structures in these rocks are cut by the cleavage and were deformed by the strain that accompanied cleavage formation. In addition, it appears that deformed calcareous concretions in the Michigamme Formation were spheroids variably flattened in bedding when tectonic deformation began. As a result, the ab plane of many ellipsoidal concretions does not coincide with cleavage. Considering all of the data, there is no evidence in these rocks or anywhere else that tectonic dewatering has been a significant factor in the formation of regional slaty cleavage.

Did the worm turn? Deformed burrow as a slump movement indicator

ABSTRACT Analysis of a deformed burrow in a slump sheet within turbidites of the Coastal Franciscan sequence of northern California leads to a slump movement direction different from that yielded by paleocurrent indicators or slump folds.