Diagenetic Origin of Continental Red Beds

Abstract

Convincing counterparts of classic ancient continental red beds, such as those of Permo-Triassic age in Europe and North Africa, and those of Pennsylvanian to Triassic age in western interior North America, are forming diagenetically today in deposits of Cenozoic age in the arid and semi-arid regions of southwestern United States and northwestern Mexico. Sequences of sediments that range from Holocene to mid-Tertiary age show critical early stages of reddening, and reveal that the pigment (hematite) originates from intrastratal alteration of iron-bearing minerals. Any type of iron-bearing mineral provides a potential source of iron for the pigment, but the most commonly occurring parent minerals are detrital ferromagnesian silicates such as olivine, augite, hornblende, biotite, etc., and detrital and authigenic iron-bearing clay minerals.

Owing to the regional aridity, the unstable ferromagnesian silicates are not destroyed by surface weathering and thus they become important constituents of the basin sediments. Those minerals which form equidimensional grains, e.g., olivine, augite, hornblende, etc., occur as accessory minerals throughout the deposits; micaceous minerals, on the other hand, e.g. biotite and clay minerals, having low settling velocities, characteristically are more highly concentrated in fine-grained, low-energy deposits. In addition, detrital clay minerals collect in the intersticies of coarser-grained, higher energy deposits, where they are mechanically infiltrated with influent surface water. Wherever any of these minerals occur, they provide sources of iron for pigment if the chemistry of the interstitial water is favorable for the formation and preservation of ferric oxides.

The sediments are not red when deposited, but they redden with time. The iron-bearing minerals are not in equilibrium with the migrating ground waters and hence they alter diagenetically (mainly by hydrolysis) and release iron which, owing to the oxidizing, alkaline nature of the interstitial water, precipitates as hematite, or a precursor oxide which converts to hematite upon aging. The mechanically infiltrated clay characteristically reddens first (within thousands to tens of thousands of years), probably because it occurs in highly permeable sediments which frequently are recharged by influent, oxygen-bearing surface water. Subsequent alteration of the framework silicates releases additional iron which results in more extensive hematite formation and more intensive reddening. Thin sections and SEM studies reveal two types of alteration of framework silicates: dissolution, and replacement by clay. Both alterations release iron to the interstitial water. X-ray analyses show that the authigenic daughter clay is mixed-layer illite-montmorillonite with 0 to 20 percent non-expandable layers, microprobe analyses show that it is ironbearing, and thin sections show that it reddens in situ. Similar clay also commonly is precipitated interstitially. These authigenic clays together with the reddened, mechanically infiltrated clay, form bright red matrix analogous to that in ancient red beds. Other authigenic minerals also commonly form at this stage of diagenesis, such as quartz, adularia, and zeolites. of these, at least quartz and adularia commonly are stained red and they contribute to the formation of the red matrix.

After prolonged intrastratal alteration (many tens of millions of years) the more unstable species of ferromagnesian silicates (e.g., olivine, augite, and hornblende) are destroyed completely, leaving no direct evidence that they originally were present. The authigenic hematite, however, unless dissolved by subsequent chemical reduction, persists indefinitely and its euhedral crystalline nature, which is best observed at highmagnification on polished surfaces or using electron microscopy, confirms its diagenetic origin.

This research has been supported by the National Science Foundation (Grant No. 33560) and the University of Colorado Council on Research and Creative Work.