VOLCANIC CONTRIBUTIONS TO MUDSTONES – AN APPRAISAL FROM A SEDIMENTOLOGICAL AND PETROGRAPHIC PERSPECTIVE

Mudstones are now commonly thought of as the combined outcome of flocculation, gravity driven flows and settling, and also bottom currents. Yet, once contributions from volcanic centers and eroding volcanic strata are considered, the proclivity of volcanic materials to undergo substantial post-depositional alteration can obscure primary sedimentological features and invite simplistic but nonetheless erroneous interpretations. For example, bentonite beds, comprised of altered volcaniclastic debris, may at first glance look like homogenous layers of clays in outcrop and be interpreted as simple event beds. In examples from the Cretaceous of Utah, however, we can show within such beds evidence for multiple successions of depositional events, for intermittent (partial or complete) erosion, bedload transport of entire thick bentonite intervals, as well as slumping and gravity flow. Making such observations is severely hampered by outcrop weathering of smectite-rich rocks, but especially “thicker” bentonites (5 cm to several meters) allow recovery of unexpanded rock if some digging is done. The original volcaniclastic grains can still be seen under the SEM in ion milled sections, and reveal these rocks to have been siltstones and even very fine sandstones at the time of deposition. In addition, even “normal” looking shales within these successions contain various proportions of altered ash bed components. In another example, mudstones from the Coya-Machali Formation (Oligocene-Miocene age) in the Chilean Andes are of reddish color and rather featureless in hand specimen. Volcaniclastic influence can be surmised via their geologic setting, but is not obvious in outcrop or drill core. When ion milled and examined by SEM, however, an abundance of silt size altered rock fragments can be attributed to a volcanic source and suggest deposition in a lacustrine setting. In conclusion, “visible” bentonite beds may record a complex history of physical transport and reworking rather than simple ash-fall events, and seemingly “normal’ mudstones may reveal “hidden” volcaniclastic contributions that can significantly influence their mechanical and weathering characteristics.