Franciscan Complex olistostrome at Crescent City, northern California

Abstract

An olistostrome and bounding turbidites are exposed within the late Mesozoic Franciscan Complex along the Crescent City (California) coastline. Facies grade in character from Mutti & Ricci Lucchi (1978) mixed facies B, C and D, to F (the olistostrome), to mixed A, B and E, progressing upwards within the Franciscan stratigraphic section. The facies F unit outcrop is up to 600 m thick and extends 12 km along strike. It consists of oblate to tabular blocks, up to 200 m in maximum dimension, of greenstone, tonalite, radiolarian chert, limestone, phyllite and greywacke dispersed in a scaly argillite matrix. The olistostromal origin of the unit is indicated by depositional contacts with bounding turbidites, by the presence of abundant recycled sedimentary clasts within the unit, and by the presence of sedimentary breccias and associated dismembered, slump‐folded turbidites both within the olistostrome and among subjacent turbidites. Sandstones are chiefly feldspathic litharenites that were very likely derived from the partially dissected, late Mesozoic Sierran‐Klamath magmatic are. Franciscan rocks record an early pervasive, layer‐parallel flattening strain in such features as extensional faults, necking and pinch‐and‐swell structures. Several scales of extensional faulting account for the juxtaposition of turbidites of different facies and/or with varying degrees of stratal disruption, the formation of sandstone lozenges, and the formation of scaly foliation in the olistostrome matrix. The latter resulted from the juxtaposition of lenticles with varying concentrations of silt and clay. These were ultimately derived from the finer divisions of turbidite beds that were incorporated into the olistostrome. The presence of gradational contacts between some sandstone olistoliths and the olistostrome matrix, and of sandstone dykes that intrude fractures and associated drag‐folded turbidite beds indicate that Franciscan sediments were not lithified during their early deformation. These sediments were deposited in either a trench or trench slope basin, and were first deformed either by gravity collapse of the trench slope cover or, less likely, by vertical loading beneath the toe of the accretionary wedge. They later were folded during internal shortening of the growing Franciscan accretionary prism.

Re-interpretation of the age and environment of deposition of Paleogene turbidites in the Andaman and Nicobar Islands, Western Sunda Arc

“…1). These rocks and the mud volcanoes are typical of melange terrenes in accretionary complexes of Eastern Indonesia (Barber et al, 1986), the Franciscan subduction zone (Worrall, 1981;Aalto, 1989), the Western Cordillera of North America (Cowan, 1985), Nias Island, Sumatra (Samuel et al, 1997), the Indo-Burma Ranges (Brunnschweiler, 1974) and the Chios, Greece and Karaburam, Turkey (Robertson and Ustaomer, 2009). While a diapiric origin has been suggested for melanges in Indonesia and Nias Island, an olistostromal (gravity flows) and/or a tectonic origin has been proposed for the other occurrences.…”

Examples of Franciscan Complex mélanges in the northernmost California Coast Ranges, a retrospective

“…Franciscan units include a shale-matrix mélange, interpreted as an olistostrome, bounded by coherent turbidites of various facies (Aalto and Murphy 1984;Aalto 1989aAalto , 1989bAalto , 1989c. Structures preserved within these rocks record early pervasive layer-parallel extension and later crustal shortening, which may be related to the growth and deformation of the Mesozoic Franciscan accretionary prism.…”

“…The mélange interpreted as an olistostrome consists chiefly of blocks of texturally and compositionally immature sandstone, with lesser amounts of radiolarian chert, pelagic limestone, greenstone, ultramafic and mafic plutonic rocks, and graphitic phyllite dispersed in an argillite matrix (Aalto 1986b(Aalto , 1989a(Aalto , 1989b(Aalto , 1989c. Accretion-related deformation has resulted in pervasive fracturing of Franciscan rocks and localized development of large contractional faults and folds.…”

“…Two types of deposits exist: sandy diamictites and shaly diamictites with scaly matrix foliation (Aalto 1989a(Aalto , 1989d. Sandy diamictites that lack matrix foliation are exposed in the northern part of Point Saint George (Figure 3(C) and (D)) and in sea cliffs due east of Preston Island (Figure 4).…”

“…Individual beds have suffered pervasive layer parallel extension (e.g. Aalto 1981Aalto , 1982Aalto , 1986bAalto , 1989aAalto , 1989b. The former contains a plumpudding assemblage of ellipsoidal blocks of oceanic plate rocks (greenstone, ribbon chert, pelagic limestone, keratophyre, tonalite, and peridotite) along with blueschist (in the Central belt Franciscan), sandstone, and conglomerate dispersed in a highly sheared shale or serpentinite matrix.…”