Abstract The chemical and physical activity of clay minerals in soils, particularly in surface horizons, is significantly mediated by interactions with organic components. And the reactivity of soil organic matter, including its resistance to decomposition, is regulated by interactions with clay minerals. This marriage of organic and inorganic soil components has profound implications for our ability to quantitatively predict the consequences of alternative soil management practices that could improve food and energy production, protect water and air quality, or mitigate the impacts of greenhouse gases on global climate. In this paper, we discuss observations of clay-humic complexes made by electron microscopy and other complementary techniques. By “clay,” we mean secondary layer silicate and oxide minerals that are common to soils. By “humic materials,” we mean soil organic matter that was derived from biological tissue but has been so transformed in the soil environment that it lacks any recognizable biological structures. Our definition of humic materials precludes any genetic or functional relationship with humic and fulvic acids, which are procedurally defined extracts of soil organic materials that we view as heterogeneous mixtures of hydrolyzed humic material, biological tissue, and black C. Data reported in this paper come from various analyses of a single smectite-rich Mollisol in Iowa. Because we investigated only one soil, we cannot generalize our results beyond saying that the studied soil was typical of smectite-rich Mollisols in the upper Midwest. Interactions between soil organic matter (SOM) and soil minerals are primarily responsible for the formation and stabilization of soil structure

ABSTRACT The upper Middle Eocene to Lower Miocene Sespe Formation is the youngest part of an ~7-km-thick Cretaceous–Paleogene forearc stratigraphic sequence in coastal southern California. Whereas Upper Cretaceous through Middle Eocene strata of southern California record a transition from local (i.e., continental-margin batholith) to extraregional (i.e., cratonal) provenance resulting from Laramide deformation (75–35 Ma), the Sespe Formation records the reversal of this process and the re-establishment of local sediment sources by Middle Miocene time. In contrast to underlying dominantly marine strata, the Sespe Formation primarily consists of alluvial/fluvial and deltaic sandstone and conglomerate, which represent terminal filling of the forearc basin. Prior to Middle Miocene dissection and clockwise rotation, the Sespe basin trended north-south adjacent to the west side of the Peninsular Ranges. The integration of paleocurrent, accessory-mineral, conglomerate, sandstone, and detrital zircon data tightly constrains provenance. Sespe sandstone deposited in the Late Eocene was supplied by two major rivers (one eroding the Sonoran Desert, to the east, and one eroding the Mojave Desert, Colorado River trough area, and Transition Zone, to the north), as well as smaller local drainages. As the Farallon slab rolled back toward the coast during the Oligocene, the drainage divide also migrated southwestward. During deposition of the upper Sespe Formation, extraregional sources diminished, while the Peninsular Ranges provided increasing detritus from the east and the Franciscan Complex provided increasing detritus from the west (prerotation). As the overall flux of detritus to the Sespe basin decreased and deposition slowed, nonmarine environments were replaced by marine environments, in which the Miocene Vaqueros Formation was deposited. The provenance and paleogeographic information presented herein provides new insights regarding the unique paleotectonic setting of the Sespe forearc from the Late Eocene through earliest Miocene. Nonmarine sedimentation of the Sespe Formation initiated soon after cessation of coastal flat-slab subduction of the Laramide orogeny and terminated as the drainage divide migrated coastward. Competing models for movement along the Nacimiento fault system during the Laramide orogeny (sinistral slip versus reverse slip to emplace the Salinian terrane against the Nacimiento terrane) share the fact that the Peninsular Ranges forearc basin was not disrupted, as it lay south and southwest of the Nacimiento fault system. The northern edge of the Peninsular Ranges batholith formed a natural conduit for the fluvial system that deposited the Sespe Formation.

The Orocopia Schist of the Orocopia Mountains is part of the regionally extensive Pelona-Orocopia-Rand Schist terrane, which is generally interpreted as a relict subduction complex underplated beneath southern California and southwestern Arizona during the latest Cretaceous–early Cenozoic Laramide orogeny. The schist in the Orocopia Mountains forms the lower plate of the Orocopia Mountains detachment fault and has an exposed structural thickness of ∼1.5 km. Prograde metamorphism occurred in the albite-epidote amphibolite facies, although the upper half of the section exhibits a strong greenschist-facies retrograde overprint. A mylonite zone just a few meters thick is present at the top of the schist. The upper plate of the Orocopia Mountains detachment fault is divided into one mappable unit consisting of Proterozoic gneiss widely intruded by 76 Ma leucogranite and a second unit dominated by anorthosite-syenite with minor amounts of leucogranite. Both the leucogranite-gneiss and anorthosite-syenite units are locally cut by faults that may be genetically related to the Orocopia Mountains detachment fault. None of the rocks in the upper plate exhibit evidence of ductile deformation related to movement on the detachment fault. 40 Ar/ 39 Ar analysis of the schist yielded total gas ages of 54–50 Ma for hornblende, 52–34 Ma for muscovite, 33–14 Ma for biotite, and 25–24 Ma for K-feldspar. A single apatite fission track sample yielded an age of 16 Ma. The above results, combined with multidiffusion domain (MDD) analysis of K-feldspar, indicate two major episodes of cooling: one beginning at ca. 52–50 Ma, the other starting at ca. 24–22 Ma. The early Cenozoic phase of cooling is attributed to subduction refrigeration combined with erosional and tectonic denudation. The greenschist-facies retrogression of the schist probably occurred at this time. The middle Cenozoic cooling event is thought to be the result of normal-sense slip on the Orocopia Mountains detachment fault. The thin mylonite at the top of the schist probably formed in association with this structure. The early and middle Cenozoic events each appear to have contributed substantially to the 30–35 km of total exhumation required to bring the schist from its maximum depth of underthrusting to the surface. Most 40 Ar/ 39 Ar ages from the upper plate fall into the following ranges: 76–69 Ma for hornblende, 75–56 Ma for biotite, and 78–42 Ma for K-feldspar. One apatite fission track age of 27 Ma was obtained from the anorthosite-syenite unit. MDD thermal histories for K-feldspar vary significantly with structural position, implying the presence of at least one major structural break within the upper plate. The distinctly old ages for the upper plate compared to the schist indicate that the former was exhumed to relatively shallow crustal levels by latest Cretaceous to early Cenozoic time. The upper plate was juxtaposed against the schist in the earliest Miocene by slip on the Orocopia Mountains detachment fault. The two-stage cooling and exhumation history for the Orocopia Schist in the Orocopia Mountains is virtually identical to that inferred recently for the Gavilan Hills to the southeast based upon a similar thermochronologic analysis. Combined with preliminary investigations of several additional bodies of schist exposed along the Chocolate Mountains anticlinorium of southeastern California and southwestern Arizona, these data provide strong evidence for a major middle Cenozoic extensional event throughout the region. The inferred middle Cenozoic extensional faults are folded by the Chocolate Mountains anticlinorium. This contradicts a recent model for erosional unroofing of the Orocopia Schist, which predicts that the Chocolate Mountains anticlinorium developed primarily during the early Cenozoic.