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Clastic dykes
Clastic dykes are sedimentary features consisting of seams of sediment truncating the surrounding host rock or deposit. These dykes can form through different processes. For example, fluidized sand can be injected through fractures, cutting through underlying and overlying deposits. Alternatively, pre-existing open cracks can be passively infilled by accumulation of a different kind of sediment. This picture shows three sub-vertical sandstone dykes, with parallel orientation, cutting through sub-horizontal mudstone and sandstone layers of the Pennsylvanian Fountain Formation, at the Balanced Rock site of Garden of the Gods Park, CO. Although different interpretations have been presented to explain the dykes in this outcrop (e.g., infill after thermal contraction of unconsolidated deposits in a cold environment), fieldwork by GRI scientists suggests that they should be interpreted as injection features, possibly related to seismic activation of small normal faults. Pencil for scale (resting vertically on right dyke).
Normal grading
Normal grading is a type of arrangement of particles in a sedimentary bed, where coarser grains lie at the bottom and finer grains at the top, resulting in a general fining-upward size distribution. Several physical processes can produce normal grading, but most commonly it is indicative of a decrease in the energy of the flow transporting the particles. The bed in the lower half of this picture shows normal grading from microconglomerate angular pebbles to granules to coarse sand at the top. Pennsylvanian Minturn Formation (CO, USA), scale in cm increments.