Development of large-scale seismites in Upper Cretaceous fluvial sandstones in a fault-proximal setting

Wizevich, Michael C., Simpson, Edward L., Hilbert-Wolf, Hannah L., and Tindall, Sarah E. (2016) Development of large-scale seismites in Upper Cretaceous fluvial sandstones in a fault-proximal setting. Sedimentology, 63 (3). pp. 1719-1738.

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Abstract

Large-scale soft-sediment deformation structures occur within fluvial sandstone bodies of the Upper Cretaceous Wahweap Formation in the Kaiparowits basin, southern Utah, USA. These structures represent an exceptional example of metre-scale fault-proximal, seismogenic load structures in nearly homogenous sandstones. The load structures consist of two types: large-scale load casts and wedge-shaped load structures. Large-scale load casts penetrate up to 4·5 m into the underlying sandstone bed. Wedge-shaped load structures include metre-scale, parallel, sub-vertical features and decimetre-scale features along the periphery of the large-scale load casts or other wedgeshaped load structures. Wedge-shaped load structures contain well-developed, medial cataclastic shear deformation bands. All load structures contain pervasive well-defined millimetre-thick to centimetre-thick internal laminae, oriented parallel to the outside form of the load structures and asymptotic to deformation bands. Both types of load structures formed because of an inverted density profile, earthquake-triggered liquefaction and growth of irregularities (a Rayleigh–Taylor instability) on the sandstone–sandstone erosional contact. The internal laminae and deformation bands formed during deformation and clearly demonstrate polyphase deformation, recording a transition from liquefied to hydroplastic to brittle modes of deformation. Decimetre-scale wedge-shaped load structures on the edge of the large-scale load casts probably formed towards the end of a seismic event after the sediment dewatered and increased the frictional contact of grains enough to impart strength to the sands. Metre-scale wedge-shaped load structures were created as the tips of downward foundering sediments were driven into fractures, which widened incrementally with seismic pulsation. With each widening of the fracture, gravity and a suction effect would draw additional sediment into the fracture. Superimposed laminae indicate a secondary syndeformational origin for internal laminae, probably by flow-generated shearing and vibrofluidization mechanisms. Large-scale and wedgeshaped load structures, polyphase deformation and secondary laminae may characterize soft-sediment deformation in certain fault-proximal settings.

Item ID: 48039
Item Type: Article (Research - C1)
ISSN: 1365-3091
Keywords: load structure; rheology; seismite; Upper Cretaceous; Wahweap Formation
Funders: American Chemical Society Petroleum Research Fund, Kutztown University Research Committee, Pennsylvania State System of Higher Education Professional Development Committee
Date Deposited: 23 Mar 2017 04:06
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040310 Sedimentology @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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