Unraveling sedimentary processes in fluvial sediments from two basalt dominated watersheds in northern Idaho, USA

Abstract

Basaltic sediments generated from the breakdown of the Columbia River Basalts (CRB) in the Lawyer Creek and Lapwai Creek watersheds of northern Idaho demonstrate that sediment composition in these fluvial systems is impacted by a number of processes, including, chemical weathering, sediment mixing, fluvial sorting, and diagenesis. Chemical weathering is documented in the compositional transformations that occur from the parent material to fluvial detritus, specifically in the leaching of labile cations (i.e., Ca, Mg, Na, and K), an overall increase in weathering indices, and the generation of secondary weathering products (i.e., clay minerals and X-ray amorphous phases) in these sediments. While the two watersheds mainly downcut through the Columbia River Basalt, the incorporation of material derived from local, glacially-derived soil and/or second cycle detritus from interbedded sandstone units significantly modifies the chemical and mineralogical signatures that would normally be used to fingerprint a mafic provenance, highlighting a potential difficulty for provenance reconstruction in sedimentary rocks derived from mixed sources. However, the influence of this mixing is less pronounced in the clay size fraction, highlighting the potential of the <2 μm sediments to preserve the chemical weathering history of the provenance during sedimentation. In addition to weathering and mixing, fluvial sorting is also recorded in the sediment composition and results in the separation of weathering products as a function of grain size. This process is first recognized from the enrichment of secondary weathering phases as grain size decreases, thus resulting in the most chemically altered material sorted into the clay size fraction of sediment deposits downstream. Lastly, early diagenesis of fluvial sediments in the CRB watersheds results in a Ce-anomaly that also correlates with grain size, suggesting the preferential formation of oxide phases in the fine-grained fraction. Furthermore, the most positive Ce-anomaly is recorded in an ancient stream deposit, suggesting that authigenic oxide precipitation initiates during sediment accumulation and advances as the deposit is cut off from the active stream channel.