Definitions and images to illustrate geological terms, links to images and website articles


3D image looking down on Point Bonita, CA - part of the Franciscan mélange - a mix of late Mesozoic metabasalts, sandstone, and conglomerates. Courtesy USGS A mélange is a mappable-sized, breccia containing varied rocks jumbled together with little continuity of contacts. The diverse blocks within a mélange are supported and separated by a matrix of fine grained material (typically shale, slate, or serpentinite) with a tectonic fabric. Mélanges originate either as components of tectonic accretionary prisms, as a result of gravitational submarine sliding (olistostromes), or through diapirism (diagram).

An olistostrome, or "gravitational mélange", is a mappable, chaotic sedimentary deposit composed of heterogeneous olistoliths (blocks) derived from submarine gravity siliding or slumping of unconsolidated sediments. Such slides may have traveled several dozen to several hundred kilometers, resulting in large, thick, heterogeneous stratiform units that accumulated somewhat chaotically from an active fault escarpment, in various tectonic settings.

Olistostromes may range from several meters up to several hundreds of meters thick, and component olistoliths (blocks)may have preserved their internal coherence to the extent that the original facies can still be established. Olistoliths are immersed in a fine-grained matrix (typically mudstone or serpentinite).

Olistostromes are mélanges formed by semi-fluid accumulation of submarine, gravitational flow. Slide masses composed of hard rocks plus semi-indurated and soft sediments fail when the softer and friable materials form a basal mobile phase. Such a slide may even have liquefied and progressively disintegrated during displacement. So, olistostromes are stratigraphic units with chaotic bedding or without true bedding, yet which are intercalated between normal sedimentary bedding sequences.

[links: images: formations: mélanges: tectonic mélange rocks; melange enclosed in dark matrix of serpentinite and containing a block of S-type, coarse granite, eastern flank of the Cordillera Occidental; a mélange is a mappable body of rock that includes fragments and blocks of all sizes, embedded in a generally sheared matrix; Franciscan mélange, another, Central Valley; Franciscan mélange - isolated blocks of resistant Franciscan rocks of various types (known as knockers) mixed into a sheared mudstone matrix; 3D of Point Bonita, Kirby Cove; mélange, Marin headlands, another, and close-up of mélange with chert clasts; close-up of disarticulated Hartland granofels block in a tectonic mélange near Cameron’s Line; mélange close-up; close-up of fragment of greenstone in mud-rich mélange, Malua Bay, south of Batemans Bay, south coast of New South Wales, fragments of sandstone in mud-rich mélange, Garden Bay, near Malua Bay, small fragments in mud-rich mélange, Sunshine Bay, south of Batemans Bay, chert mélange from the southern side of Burrewarra Point, south of Batemans Bay; exotic breccia blocks, eroded from the Hoh mélange, near Goodman Creek, 2; mélange, Kiryu-Kurohone Complex, Japan; tectonic mélange at the base of a large thrust sheet of Bravo Lake volcanic rocks; quartzite blocks in Gwna Mélange, Llyn Peninsula and Anglesey, Wales, and pillow-basalts, Porth Dinllaen ; Dunnage Mélange near Gander, Newfoundland; mélange, Nfld; Cretaceous rock in mélange in CA; thrust fault composed of imbricated structure, coherent and mélange facies, shear fabrics, Japan; amphibolite-grade mélange in Catalina Schist; olistrosomes: olistrosome; olistostromes (brownish strata in lower part of hillside) and ophiolites (dark material in upper part of section) exposed in eastern Cuba; webpages: trenches and mélanges]

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