Pebble fabric in braided stream deposits with examples from Recent and „frozen" Carboniferous channels (Intrasudetic Basin, Central Sudetes).

Abstract

This paper deals with the methodology and inter pretation of pebble fabric measurements in ancient and some Recent fluvial conglomerates and gravels. The field data came mostly from the Intrasudetic Basin (Central Sudetes, SW Poland) where the fluvial valley-floor conglomerates of the Lower Carboniferous age were investigated in detail; some modern pebbly streams were also inspected. Pebble fabric was studied primarily in „frozen” channels and fossil bar features. Systematically measured were only dip azimuths of the AB planes of platy pebbles. Fabric measurements were as a rule closely integrated with detailed sedimentological investigations into deposits enclosing and surrounding the gravels studied. It is evident from the field data that not uncommonly are directions suggested by prominent maxima in fabric diagrams at an angle to the real (paleo)flow, or even the two directions are 90° apart. Inclined pebbles may also occur that are, in fact, not imbricated. Furthermore, in many two -or three-maximum diagrams the bisectrix of ananagle between these maxima is significantly deviated from the actual (paleo)flow. There are at least eight possible fabric patterns in the Kulm conglomerates in question. These are found to depend on (and to reflect) both the channel sinuosity and the position of a given gravel investigated with respect to the channel axis and banks. In other words the arrangement of pebbles in an alluvial channel is a bank-controlled phenomenon. This long recognized, but apparently underestimated principle was used by the author in order to recognize and reconstruct in apparently „featureless” fluviatile gravels of original channel, and bar features. Furthermore, in some in stances it provides reasonable criteria for distinguishing between deposits of low- and high-sinuosity reaches, too. The observations all point to the conclusion that any interpretation of pebble fabric measurements needs information about the original bar-and-channel topography, channel symmetry, and the position of a conglomerate investigated with respect to the original channel- and bar features. The aim of this study is also to demonstrate that fabric measurements not sup ported and not confirmed by another sedimentological observations may lead to serious paleogeographic misinterpretations and should be eliminated from the geologic practice. On the other hand, only such integrated investigations may provide a new environmental framework for more reasonable interpretations of pebble fabric diagrams to be made. In other words fabric diagrams should not be interpreted and classified in terms of „diagram symmetry", but they certainly ought to be analysed in terms of symmetry of the original depositional milieu including both channel-and bar topography and flow pattern. Finally, there is growing evidence that gravels originated under quite different environmental conditions may display identical fabric patterns. There fore in the light of our present knowledge, diagram patterns alone do not provide reasonable (nor sufficient) criteria for any environmental interpretation of gravels they are found in.