SEPM Strata


*arkosic sandstone*
igneous rocks metamorphic rocks minerals sedimentary rocks	QFL Composition Description A sandstone with a mixture of quartz and orthoclasefeldspar (pink). The sand grains stand out in high relief in this specimen (i.e. there is little matrix) placing this in a sand category. The sand grains here are very large, some even drifting over into the granule size range (2-4 mm) which would technically make this a gravel (conglomerate). We put it here since most of the grains fall into the sand category. Tectonic Association Arkoses with abundant feldspar such as this come from the weathering of a sourceland rich in Alkali Granite. The absence of virtually any other grains other than quartz and feldspar indicate the immediate source of this rock was not a mixed sourceland. Formation & Environments A sandstone this clean (i.e. without matrix) indicates a source where chemical weathering was not extreme. The course grain size and relatively good sorting of this sediment implies water transportation, probably in a braided river system. Detail QFL Composition Description The sand grains here are very large, some even drifting over into the granule size range (2-4 mm) which would technically make this a gravel (conglomerate). We put it here since most of the grains fall into the sand category. In this closeup the quartz and orthoclase grains stand out individually. Notice the absence of matrix among the grains; you can see down the sides and almost under the grains, an indication of little or no matrix.