Origin of Pebbly Mudstones

Research Article| August 01, 1957 ORIGIN OF PEBBLY MUDSTONES JOHN C CROWELL JOHN C CROWELL DEPARTMENT OF GEOLOGY, UNIVERSITY OF CALIFORNIA, LOS ANGELES 24, CALIFORNIA Search for other works by this author on: GSW Google Scholar GSA Bulletin (1957) 68 (8): 993–1010. https://doi.org/10.1130/0016-7606(1957)68[993:OOPM]2.0.CO;2 Article history received: 08 Oct 1956 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation JOHN C CROWELL; ORIGIN OF PEBBLY MUDSTONES. GSA Bulletin 1957;; 68 (8): 993–1010. doi: https://doi.org/10.1130/0016-7606(1957)68[993:OOPM]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Massive marine mudstones containing scattered pebbles crop out at several places in California in strata ranging from Jurassic to Pliocene. Similar rocks, also characterized by unsorted lithology with widely dispersed pebbles, are exposed in Switzerland in Wildflysch strata (Eocene) and in Bavaria in Cenomanian beds.The California pebbly mudstones are found as lenticular beds in association with slump structures, including slump overfolds. They are interbedded with graywacke, mudstone, conglomerate, and sandstone that were in part laid down by turbidity currents as shown by graded bedding, load casts, current bedding, and other primary structures. Here at places successive stages illustrating the origin of the pebbly mudstones are preserved. Beds of graded conglomerate, laid down on soft water-saturated mud, became unstable, slumped downslope, and mixed pebbles with mud.Pebbly mudstones originating by this process resemble many rocks designated as tillites. In fact, some glacial episodes may have been introduced into geologic history based on the interpretation of such rocks as tillites without independent supporting evidence. Such “tillites” should therefore be restudied, for they may have originated by slumping and mixing of interbedded gravel and mud penecontemporaneously with deposition.Small-scale slump structures and mixed rocks, such as those described here, are considered as part of a continuous series which ranges upward from turbidity currents through slumps and large slides to huge tectonic units at the upper extreme. All these phenomena owe their origin to downslope movement under gravity. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.