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Origins of Bimodal Stratigraphy In Fluvial Deposits: An Example From the Morrison Formation (Upper Jurassic), Western U.S.A.
Changes in stacking density of fluvial channel-belt sand bodies are one of the most obvious aspects of alluvial architecture. The proximity of sand bodies to each other has been used as a field criterion to suggest major changes in external factors, such as basin subsidence rates, relative changes in base level, and changes in climate. In places rapid changes in stacking density are found in vertical succession between very high to very low net sandstone. In the absence of an intervening unconformity such changes are not the result of two unrelated, but juxtaposed, fluvial regimes but represent a more transitional change of formative conditions or preservation style.
To study whether a rapid change in stacking density was controlled by external factors rather than autogenic behavior, we sought to document other possible changes in deposition that took place in concert with changes in stacking pattern. The Morrison Formation, Upper Jurassic of eastern Utah and western Colorado, U.S.A., contains a rapid transition from the Salt Wash Member, with an average 63% net sand, to the overlying Brushy Basin Member, dominated by muddy floodplain deposits (10% net sand). We measured nine sedimentary characteristics related to fluvial system properties in both members along a down-paleoflow-oriented transect. The only significant change found between the two members is the composition and abundance of floodplain clay minerals. The transition in stacking density is coincident with a rapid increase in smectitic clay, interpreted to result from increased volcanic ash influx, at the expense of illitic clay. The ash was preferentially accumulated in the floodplain, either as airfall or carried in the wash load of rivers.
Geometric calculations suggest that in order to generate the observed change in stacking pattern by the addition of volcanic ash alone, more than half of the flood-plain deposition would have to be composed of ash. An increase in ash supply would increase floodplain sedimentation rate and reduce the frequency of avulsion. Together these effects could reduce the stacking density by the requisite 50%.
Dependence of stacking density on floodplain sedimentation rate highlights the danger in assuming changes in stacking density observed in alluvial sequences are mainly controlled by other external factors such as sea-level fluctuation, subsidence rate, or overall aggradation rates.
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