Theodore R Walker

Formation of Red Beds in Modern and Ancient Deserts

New evidence supports the theory that the hematite pigment in many red beds, particularly those associated with evaporites and aeolian sandstones, formed after deposition in hot arid or semiarid climates. The evidence includes field, petro-graphic, and chemical data collected from studies of two stratigraphic sequences that contain red beds: (1) Recent, Pleistocene, and Pliocene deposits in the Sonoran desert of northeastern Baja California, Mexico, and (2) late Paleozoic deposits in Colorado. The sequence in the Sonoran desert contains examples of red beds forming today in a hot dry climate. They are associated with bedded evaporites and occur where regional faunal, floral, and pedological evidence indicates rainfall has been low throughout the postdepositional history of the sediments. The facies associations reflect deposition in fluvial and fluvial-marine transitional environments. Red arkose fanglomerates occur on the flanks of the highland source areas, and red muds, probably of intertidal and shallow subtidal origin, occur in the transition sediments. Both of these red-stained facies show progressive stages of in situ alteration of nonred sediments to hematite-stained red beds, and in each, the iron in the stain is derived from intra-stratal alteration of iron-bearing detrital grains, particularly iron silicates such as hornblende and bio-tite. The late Paleozoic red beds of Colorado contain rock types and facies associations similar to those of the Sonoran desert; they are interpreted as ancient counterparts of those red beds. Numerous lines of field and petrographic evidence indicate that the hematite pigment in the late Paleozoic red beds formed in place after the sediments were deposited in desert basins.

Diagenesis in first-cycle desert alluvium of Cenozoic age, southwestern United States and northwestern Mexico

Petrographic studies of first-cycle desert alluvium of Cenozoic age in the southwestern United States and northwestern Mexico show that the mineralogy, texture, and chemical composition of the deposits have been changed diagenetically. The mineralogy has been changed by addition of mechanically infiltrated clay, partial removal of framework grains of feldspars and ferromagnesian silicates, and precipitation of authigenic potassium feldspar, zeolite, montmorillonite, quartz, hematite, and calcite. The texture has been changed by three processes: (1) infiltration of detrital clay and formation of authigenic montmorillonite, which form interstitial clayey matrix not present in the original sediment, (2) formation of voids where framework grains have been dissolved, and (3) in situ formation of silt and other fine-grained sizes. The chemical composition has been changed by infiltration of clay minerals that are richer in aluminum and lower in alkalis and alkaline earths than the original sediment and by removal in ground water of some of the ions released by dissolution and replacement of framework grains. These changes have significantly increased the mineralogical maturity and decreased the textural maturity of the sediments diagenetically. Four major conclusions are drawn from the studies. (1) Some or all of the diagenetic alterations observed in these deposits probably occurred in many analogous ancient first-cycle alluvial deposits at a comparable time in their history. (2) Prolonged movement of ground water through first-cycle deposits may cause unstable minerals to be removed completely, or nearly so, leaving no direct evidence that they were important original constituents of the deposits. (3) Ancient first-cycle alluvium probably rarely, if ever, has the same mineralogy, texture, or chemical composition that the sediments had when deposited. (4) The present mineralogy, texture, and chemical composition of ancient first-cycle alluvial deposits probably do not accurately reflect lithology and climate in the source area or the nature of depositional currents and other environmental factors in the depositional basin.

Diagenetic Origin of Continental Red Beds

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.

Development of Chemical Remanent Magnetization during Early Stages of Red-Bed Formation in Late Cenozoic Sediments, Baja California

Petrographic, rock magnetic, and paleomagnetic studies of fine-grained red sediments of late Cenozoic age in Baja California show that the sediments have variably acquired chemical remanent magnetization (CRM) overprints that have obscured the original magnetization of the deposits. The chemical remanence in the sediments is carried predominantly by three authigenic minerals — hematite, goethite, and a Mn · Ba compound, herein called “hydropsilomelane” — that occur as pigments and as concretions. Remanence directions generally associated with each mineral are goethite, reverse; hydropsilomelane, normal; and hematite, normal or reverse. Some samples possess a remanence that is strong and normal or strong and reverse; these generally contain only one of the authigenic minerals in abundance. Many samples, however, are weak in intensity and random in direction. When such samples are split into parts and measured, it is generally found that each part is strongly magnetized but that some parts are normal and others are reverse in direction. In such cases, the magnetization of the whole sample is resultant of the multiple components that are generally carried by two or more of the authigenic minerals. The following conclusions can be drawn concerning the acquisition of CRM in the Baja California deposits: (1) The sediments contain chemically unstable iron- and manganese-bearing minerals, such as hornblende and biotite, that are susceptible to postdepositional alteration, and they have provided the parent material for the authigenic magnetic minerals. (2) Authigenic magnetic minerals, growing from crystallites, generally acquired a remanence that was parallel to the Earth9s field when they surpassed the critical grain size. (3) The rate of CRM acquisition has not been uniform in these sediments, probably because the processes of alteration and formation of authigenic magnetic minerals depend on an interplay of highly variable factors such as the chemistry of the interstitial water, hydraulic gradients, and mineralogy of the sediments. (4) Complex variability in acquisition of CRM has led to a remanence stratigraphy that bears little discernible correlation with the geomagnetic field at the time of deposition.

Formation of Red Beds in Moist Tropical Climates: A Hypothesis

Studies of alluvial deposits of Holocene and Pleistocene age in Puerto Rico and in The Orinoco basin suggest that red beds can form diagenetically in moist tropical climates by intrastratal alteration processes that are similar to those producing red beds diagenetically in deserts. The studies show that modern sediments from these tropical regions contain ample iron, in the form of both detrital oxides and unstable iron-bearing silicates, to produce bright red sediments if the interstitial chemical environment is favorable for the formation and preservation of iron oxide. That such favorable environments exist in some tropical regions is indicated by the fact that yellow and brown iron oxides commonly occur in Quaternary alluvial deposits in Puerto Rico many tens of feet below the water table. In addition, chemical analyses of ground-water samples from Puerto Rico show that the interstitial environment in the alluvium commonly lies in the stability field of hematite. These data indicate that sediments which may be precursors of red beds are accumulating in Puerto Rico today. Where favorable interstitial chemical conditions persist long enough for the yellow and brown oxides (both detrital and authigenic) to be converted to red oxides, these deposits should produce red beds. Such deposits may be modern analogues of ancient red beds associated with fauna, flora, or other evidence of warm, moist conditions at the time of deposition. Chemical and mineralogic analyses of samples from Puerto Rico and the Orinoco basin, when compared with analogous data from the Sonoran Desert, do not reveal any criterion that can be confidently extrapolated to ancient sediments to differentiate red beds formed in moist climates from those formed in deserts. Other characteristics of ancient red beds, such as fauna, flora, and association with evaporite minerals or with aeolian sandstone, provide the most reliable evidence of the climate at the time of deposition.

Iron Content of Modern Deposits in the Sonoran Desert: A Contribution to the Origin of Red Beds

Surface weathering on the Sonoran desert produces iron-bearing clay minerals which are concentrated in fine-grained sediments eroded from the desert. The clay fraction of the desert soils and desert-derived alluvium contains an average of about 4.5 percent total iron. An average of less than 1.0 percent iron occurs in oxide coatings on grains; the remainder is held in the clay-mineral lattices. It is inferred that under favorable interstitial chemical conditions the iron oxide coatings age to hematite and the clay undergoes postdepositional alteration, yielding additional iron which ultimately forms additional hematite pigment. Biotite, another important source of iron, commonly is associated with the clay and undergoes similar intrastratal alteration. It is concluded that the characteristic concentration of iron and hematite pigment in mudstones and shales in many ancient red beds, particularly in red beds that are associated with evaporites or aeolian sandstones, or both, reflects initial concentration of iron-bearing clay minerals and biotite in fine-grained sediments derived from desert source areas.

COLOR OF RECENT SEDIMENTS IN TROPICAL MEXICO: A CONTRIBUTION TO THE ORIGIN OF RED BEDS

The color of Recent fluvial sediments in tropical Mexico supports the hypothesis that the hematite pigment in ancient red beds forms in situ after deposition and is not derived from erosion of red tropical (lateritic) soils, as is commonly claimed. Although red soils do occur in the region9s hot humid source areas, the Recent alluvium that is transported and deposited by rivers is grayish brown because the red detritus is masked by more abundant nonred material. Red coloration occurs only in soils resulting from prolonged weathering on well-drained upland surfaces of Pleistocene age or older.

Reversible Nature of Chert-Carbonate Replacement in Sedimentary Rocks

Textural relationships observed in thin sections of chert-bearing carbonate rocks commonly indicate that chert, which originated by replacement of the carbonate host rock, has then been replaced by younger carbonate minerals. The principal evidences of such replacement reversals are: (1) localization of carbonate minerals along and adjacent to fractures in chert, (2) transection of authigenic siliceous textures by carbonate minerals, and (3) occurrence within carbonate minerals of inclusions that have been inherited from chert. In some chert-bearing carbonate rocks the textural relationships indicate that multiple replacement reversals have occurred between chert and carbonate minerals. The cause is not known, but data suggest that the reversals may occur in response to variations in pH where interstitial water is highly alkaline (above pH 9) or in response to variations in temperature where sediments have been deeply buried.

CARBONATE REPLACEMENT OF DETRITAL CRYSTALLINE SILICATE MINERALS AS A SOURCE OF AUTHIGENIC SILICA IN SEDIMENTARY ROCKS

Petrographic studies show that carbonate replacement of detrital crystalline silicate minerals is an important and widespread process in sedimentary rocks. Investigations of three different rock suites indicate that authigenic siliceous features commonly occur in or near strata that contain partially to completely replaced silicate grains. This association implies a genetic relationship and suggests that silica released by carbonate replacement may be an important source of authigenic silica in some sedimentary rocks. Such replacement is probably more common in sedimentary rocks than has been suspected because completely replaced grains commonly leave no evidence of their original presence in the sediment, or the replaced grains are preserved as carbonate pseudomorphs that can be mistaken for grains of clastic carbonate. Even in occurrences containing relicts of incompletely replaced silicate grains it generally is not possible to determine to what extent replacement has taken place. The mechanism of solution, migration, and reprecipitation of silica is uncertain. Earlier writers have suggested that carbonate replacement of opal and subsequent reprecipitation of the released silica occurs in response to pH variations. If so, the replacement and reprecipitation of silica from crystalline silicates might be similarly explained. Recent data, however, cast doubt on this theory.

GROUND-WATER CONTAMINATION IN THE ROCKY MOUNTAIN ARSENAL AREA, DENVER, COLORADO

Improper waste-disposal practices at the Rocky Mountain Arsenal have seriously damaged the free ground-water aquifer throughout an area of approximately 6½ square miles. Contaminants include chlorates and 2,4-D-type compounds, both of which are effective herbicides. Contaminated ground water within the affected area is toxic to agricultural crops and unpotable for humans. Corrective measures have been taken to halt further contamination, but the area of toxicity is expanding owing to migration of the body of ground water already contaminated.