Short-term sediment accumulation directly on basalt flows without the damming of drainage is not well studied. Can lakes form on basalt surfaces during times of volcanic quiescence between outpourings? Large faulted downthrown blocks of the lower Jurassic Kalkrand basalt within the Hardap Recreational Resort in central Namibia provided depressions for sediment accumulation during two periods of volcanic quiescence. The sedimentary infill was preserved as two sedimentary interlayers from 6 to 8 m in thickness between lava flow units. These sedimentary rocks, formerly interpreted as dominantly lacustrine deposits, now can be recognized as alluvial fill because of the presence of pedogenic mud aggregates. Sedimentation processes filling these fault-bound depressions included sheetflooding and grain flow, archived in successions containing sandstone, siltstone, and mudrock. Large cracks and fractures on the basalt surfaces, filled with quartz sandstone, showed that the regional water table was below the basalt flow surface, making the formation of lakes impossible, especially because of the permeability and porosity of such flows in general. No pattern or cyclicity during sediment deposition was found and paleocurrent measurements showed random sedimentary influx directions, probably related to the lava field surface topography. Autochthonous weathering of basalts added to the allochthonous drainage input of quartz-rich sand and silt that produced sandstone and siltstone containing smectitic clays. Most of these clay-rich siltstones and sandstones contained mud laminae within traction-load sedimentary structures, providing evidence for the presence of pedogenic mud aggregates within these sheetflood deposits. Geothermal groundwaters circulating through the basalt and the sediments, mixing with meteoric waters, produced Ca-rich diagenetic waters with an elevated temperature to precipitate calcite cement and dendritic crystals. This work demonstrates that characteristics of mud deposition can provide important clues in determining the true depositional paleoenvironment of these Jurassic sedimentary interlayers between basalt flows.

Nonpedogenic calcrete is difficult to distinguish from pedogenic calcrete in the fossil record; both alpha and beta textures have been observed from fossil and modern examples. However, a calcrete from the New Haven Arkose (Hartford Basin, Connecticut) is shown here to be of a nonpedogenic origin through sedimentologic and petrographic evidence. An accumulation of thin sheets of displacive calcite layers found in a decimeter-thick horizon of anastomosing veins within the upper portion of a red mudstone is correlated to calcite cement found in the overlying sandstone. Based on petrography, we recognize six generations of calcite in the mudstone-sandstone hosts. The first five generations are associated with rhizoliths that can be related to deep taproots and are interpreted to have formed by precipitation from shallow groundwater. There are no vadose-type cement morphologies; the calcite has luminescent zones, indicating that Mn was soluble and thus oxygen levels were low. These cements clearly formed several meters below what would have been the surface of the channel sand body. We suggest that calcite cement stratigraphy combined with redox models for the behavior of Mn (as well as Fe and U) may aid in the identification of nonpedogenic versus pedogenic carbonates in the geologic record. Additionally, the calcite from this carbonate layer has been dated using the U-Pb method. Our results provide insight into the environmental and diagenetic fluid conditions favorable for providing a spread in U/Pb ratios that are suitable for precise dating of calcites in otherwise undateable sections. Las calcretas no-pedogénicas son difíciles de diferenciar de las pedogénicas en el registro antiguo; los dos tipos de textura alfa y beta se reconocen en depósitos antiguos y recientes. Sin embargo, en el caso concreto de la calcreta de New Haven Arkose (Cuenca Hartford, Connecticut), los datos sedimentológicos y las evidencias petrográficas permiten demostrar su origen no-pedogénico. La presencia, dentro de lutitas rojas, de finas capas de calcita desplazativa dentro de un horizonte decimétrico de venas anastomosadas se puede correlacionar con la formación de cemento calcítico en las areniscas infrayacentes a dichas lutitas rojas. Petrográficametne se reconocen seis generaciones de calcita en las lutitas y areniscas que constituyen el sustrato. Las cinco primeras generaciones están asociadas con rizolitos de sistemas radiculares profundos y se pudieron formar por precipitación a partir de aguas freáticas someras. No hay cementos vadosos y la calcita tiene zonas luminiscentes indicando que el Mn era soluble y, por tanto, los niveles de oxígeno bajos. Estos cementos se formaron claramente varios metros por debajo de lo que fue la superficie del canal de arenas. Sugerimos que la estratigrafía de los cementos de calcita combinada con los modelos redox de comportamiento de Mn (también Fe y U) pueden ayudar en la identificación de los carbonatos pedogénicos y no-pedogénicos en el registro geológico. Además la calcita de esta capa carbonática se ha datado por el método de U-Pb. Nuestros resultados permiten una mejor caracterización de las condiciones ambientales y de los tipos de fluidos más favorables para ampliar el rango de valores U/Pb necesarios para datar calcitas en secciones que de otro modo no se podrían datar.

Abstract Pedogenic mud aggregates are sand-size particles composed of clay flakes and silt grains, produced in vertic soils with expandable clays during seasonal cycles of wetting and drying. The aggregates are commonly reworked from floodplain soils and transported as bedload, but they tend to be destroyed by compaction during burial. Their identification is important for interpreting floodplain dynamics, i.e., bedload transport rather than suspension settling of clay. Expandable clays, principally smectite, are necessary for aggregate formation, and are commonly produced during weathering of mafic volcanicrocks and ash, which are prominent constituents of many rift-basin fills. To date, pedogenic mud aggregates have been documented from some ancient rift fills. A new occurrence of mud aggregates is documented from the upper part of the Triassic-Jurassic New Haven Arkose of the Hartford rift basin, Connecticut. The sand- and silt-size aggregates are present in laminated and cross-laminated sandstones and siltstones, where the larger framework grains have protected the aggregates from destruction during compaction. Blue-light microscopy was especially helpful in aggregate identification. The host strata represent shallow bedload channels and floodplain sandstone sheets interbedded with thin red mudstones, interpreted as immature paleosols, and the aggregates are inferred to have originated principally from pedogenic activity in the mudstones and/or other adjacent paleosols. The high-energy floodplains were frequently reworked by overbank floods, and some mudstones may have been deposited by active flows capable of bedload transport of aggregates. Mobilization of soil aggregates is probably promoted in rift settings similar to that of the Hartford basin, where floodplains are small and frequently reworked by high-discharge events.