Clay Mineral Formation

Record of palaeoclimate across theCretaceous–Palaeogeneboundary from palaeosols in the west‐central San Juan Basin, New Mexico, USA

“…The San Juan Basin of north‐western New Mexico contains an extensive sedimentary record that extends above and below the K–Pg boundary. In contrast to the lower Palaeocene palaeosols of the Nacimiento Formation, which have been well‐studied in the west‐central San Juan Basin (Davis et al, 2016; Hobbs, 2016; Hobbs & Fawcett, 2022), the late Maastrichtian palaeosols of the Naashoibito Member of the Ojo Alamo Formation have been described in only general terms (Lehman, 1985). This study provides new, detailed descriptions of palaeosols of latest Maastrichtian age in the San Juan Basin for the specific purpose of interpreting general palaeoclimate conditions prior to the K–Pg boundary event.…”

“…This basic guideline comes with the caveat that the clay mineralogy of the soil may reflect the composition of the source rocks for the sediment rather than the weathering conditions in the depositional basin (cf. Chamley, 1989; Chesworth et al, 2016; Evans, 1992; Spinola et al, 2017; Velde & Meunier, 2008). Thus, immature palaeosols, for example, Protosols, may inherit a substantial portion of their clay mineralogy from the source area due to the shorter period of residence for mineral grains in the A horizon, whereas mature palaeosols, in which the soil materials spend longer periods in the active soil layer, may better reflect climate conditions in the depositional basin.…”

“…The base of the Nacimiento Formation is placed at the basal mudstone overlying the uppermost of these interbedded sandstones. The Nacimiento Formation consists mainly of pale blue to greenish‐grey (drab) coloured mudstones, sandy mudstones and muddy sandstones with lesser, variegated olive‐green to purple and mottled red‐grey mudstones (Baltz et al, 1966; Davis et al, 2016; Hobbs, 2016; Hobbs & Fawcett, 2022; Rains, 1981; Williamson & Lucas, 1992). The lower part of the Nacimiento Formation is designated the Arroyo Chijuillita Member (Williamson & Lucas, 1992).…”

“…The lower part of the Nacimiento Formation is designated the Arroyo Chijuillita Member (Williamson & Lucas, 1992). Sandstone bodies typically display lateral accretion surfaces and are thus interpreted as recording the transition to high‐sinuosity fluvial systems, in contrast to the low sinuosity of the underlying Kimbeto Member streams (Davis et al, 2016). One of the more distinctive features of the Nacimiento Formation is the occurrence of numerous thin, resistant ledges that are siliceous in composition (Davis et al, 2016; Hobbs, 2016; Rains, 1981; Williamson & Lucas, 1992).…”

“…Sandstone bodies typically display lateral accretion surfaces and are thus interpreted as recording the transition to high‐sinuosity fluvial systems, in contrast to the low sinuosity of the underlying Kimbeto Member streams (Davis et al, 2016). One of the more distinctive features of the Nacimiento Formation is the occurrence of numerous thin, resistant ledges that are siliceous in composition (Davis et al, 2016; Hobbs, 2016; Rains, 1981; Williamson & Lucas, 1992). These beds are termed silcretes, the term used to describe silica‐cemented sedimentary layers in a broad sense by most authors.…”