The significance of angular K-feldspar grains in ancient sandstones

Abstract

In our studies of ancient sandstones, many of which are purported in the conventional literature to be eolian deposits, we frequently encountered angular K-feldspar sand grains. In particular, we encountered them while studying the Coconino Sandstone of Arizona, but we have found them in many other ancient sandstones as well. To gain some insights on the petrology of ancient "eolian" sandstones, we studied the petrology of a number small ergs in the western United States, beach and dune sands along the California and Oregon and reviewed the literature on the petrology of modern eolian and subaqueous deposits.In our literature review and from our own observations along the California and Oregon coastlines, we found that fluvial and shoreline processes are not sufficient to cause rounding of sand grains of any type, even after energetic and prolonged longshore transport and frequent tidal activity. Conversely, when sand grains are picked up by eolian processes and transported to coastal dunes, all species of mineral grains are quickly rounded, even over short distances. K-feldspar is rounded faster than quartz probably because it is softer and cleaves easier. We frequently encountered rounded K-feldspar grains in the small ergs we examined despite many of them being close in proximity to sources of angular K-feldspar sand grains. In larger ergs, all types of sand grains become quickly rounded and angular grains only occur if there are local fluvial or coastal sources for them.The frequent occurrence of angular K-feldspar grains that we found in ancient cross-bedded sandstones, purported to be made by eolian processes, causes us to question whether these deposits were made by eolian activities or not. The presence of angular K-feldspar may be one petrographic criterion for identifying ancient fluvial and marine deposits. The goal of this paper is to document the ubiquitous occurrence of angular K-feldspar grains in many supposed ancient cross-bedded sandstones. Coupled with other criteria, angular K-feldspar sand grains are a crucial piece of data that might be used to argue that these ancient sandstones were formed by aqueous rather than eolian processes.

The significance of micas in ancient cross-bedded sandstones

“…There are many other criteria besides mica to consider when determining an environment of deposition for cross-bedded sandstones. One of these, angular K-feldspar, is addressed by Whitmore and Strom (2018). We do not think it is a coincidence that many of our samples had both angular feldspars and mica grains.…”

The Coconino Sandstone (Permian, Arizona, USA): Implications for the origin of ancient cross-bedded sandstones

“…Surprisingly, it is often more angular than the harder quartz sand (orthoclase = Mohs 6, quartz = Mohs 7). Alos see Whitmore and Strom (2018).…”

“…This trace consists of two parallel rows of evenly spaced V-shaped prints and lacks a medial impression. Whitmore and Strom (2018) have published a table summarizing the literature on many of these sandstones (see their Appendix I).…”