The significance of micas in ancient cross-bedded sandstones

Abstract

The cross-bedded Coconino Sandstone is almost certainly within the stratigraphic range of the Flood, however it is commonly cited by conventional geologists as the classic example of an eolian deposit, and thus an argument against the scientific viability of the Flood. In our petrographic study of the Coconino Sandstone, we discovered muscovite mica (and sometimes biotite mica) in almost every thin section. This is surprising given that micas have not previously been reported in this, or any, "eolian" cross-bedded deposit. The mica found is detrital in character (i.e., it is not an alteration product) and thus is part of the primary depositional fabric. This led to the investigation of other cross-bedded sandstones from around the world, especially those of similar stratigraphic age, all of which have been conventionally interpreted as wholly or partly eolian-the same frequent occurrence of micas was observed. Previous laboratory experiments have provided some framework for understanding this discovery. Based on those experiments, it was found that mica cannot survive continuous transport much more than four days (or about 500 km) by simulated eolian processes, but can last for more than a year (or about 7,500 km) when transported continuously by simulated subaqueous processes. Field observations confirm that modern ergs contain virtually no micas, of any size, except in cases where mica sources (such as granite outcrops, beach sand or fluvial sand) are located in the immediate vicinity (~<10 km) of the erg. By contrast, the Coconino sand body and its correlative stratigraphic units stretch for many hundreds of kilometers across (with a total area of 2.4 million km 2), and therefore the interior of the deposit should be virtually mica-free if formed by eolian processes. We catalog and illustrate a large number of cross-bedded sandstones that contain mica grains (mostly muscovite) as an accessory mineral. The dominant conventional view is that these sandstones are eolian in origin, but experimental data and field observations suggest otherwise. The presence of micas in cross-bedded sandstones is a previously neglected criterion that can be used to argue for a subaqueous depositional environment for the formation of cross-bedded sandstones. KEY WORDS experimental mica abrasion, cross-bedded sandstones, muscovite, biotite,

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

“…The presence of muscovite flakes in nearly every thin section of Coconino that we studied ( Fig. 16) strongly argues for an aqueous origin of the deposit (see Borsch et al 2018 in these proceedings).…”

The significance of angular K-feldspar grains in ancient sandstones

“…We think the zircon evidence is compelling and does suggest a distant origin for some of the Coconino sand. However, based on the angular K-feldspar and mica (Borsch et al 2018), we feel that some type of aqueous transport was primary. Any eolian transport would have quickly rounded the K-feldspars and caused the micas to disappear.…”