Processes of magma/wet sediment interaction in a large-scale Jurassic andesitic peperite complex, northern Sierra Nevada, California |
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Authors: | Richard E Hanson Ulysses S Hargrove |
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Institution: | (1) Department of Geology, Texas Christian University, P.O. Box 298830, Fort Worth, Texas 76129, USA Fax: 817–257–7789, e-mail: hanson@gamma.is.tcu.edu, US |
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Abstract: | The Middle Jurassic Tuttle Lake Formation in the northern Sierra Nevada, California, comprises a thick volcaniclastic sequence
deposited in a submarine island-arc setting and penetrated by numerous related hypabyssal intrusions. A composite andesite-diorite
intrusive complex ≥4.5 km long and ≥1.5 km thick was emplaced while the host Tuttle Lake sediments were still wet and unconsolidated.
Large parts of the intrusive complex consist of peperite formed where andesitic magma intruded and intermixed with tuff, lapilli-tuff
and tuff-breccia. The southern half of the complex consists of augite-phyric andesite containing peperite in numerous small,
isolated pockets and in more extensive, laterally continuous zones. The peperites comprise three main types recognized previously
in other peperite studies. Fluidal peperite consists of small (≤30 cm), closely spaced, at least partly interconnected, globular to amoeboid andesite bodies enclosed
by tuff. This peperite type developed during intrusion of magma into fine-grained wet sediment along unstable interfaces,
and fluidization of the sediment facilitated development of complex intrusive geometries. Blocky peperite and mixed blocky and fluidal peperite formed where magma intruded coarser sediment and underwent variable degrees of brittle fragmentation by quenching and dynamic
stressing of rigid margins, possibly aided by small steam explosions. The northern half of the intrusive complex consists
predominantly of a different type of peperite, in which decimetre-scale plagioclase-phyric andesite clasts with ellipsoidal,
elongate, or angular, polyhedral shapes are closely packed to widely dispersed within disrupted host sediment. Textural features
suggest the andesite clasts were derived from conduits through which magma was flowing, and preserved remnants of the conduits
are represented by elongate, sinuous bodies up to 30 m or more in length. Disruption and dispersal of the andesite clasts
are inferred to have occurred at least partly by steam explosions that ripped apart a network of interconnected feeder conduits
penetrating the host sediments. Closely packed peperite is present adjacent to mappable intrusions of coherent andesite, and
along the margin of a large mass of coarse-grained diorite. These coherent intrusions are considered to be major feeders for
this part of the complex. Examples of magma/wet sediment interaction similar in scale to the extensive peperites described
here occur elsewhere in ancient island-arc strata in the northern Sierra Nevada. Based on these and other published examples,
large-scale peperites probably are more common than generally realized and are likely to be important in settings where thick
sediment sequences accumulate during active volcanism. Careful mapping in well-exposed terrains may be required to recognize
large-scale peperite complexes of this type.
Received: 8 June 1998 / Accepted: 4 December 1998 |
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Keywords: | Peperite Magma/sediment interaction Jurassic island-arc deposits Northern Sierra Nevada |
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