Clay clasts

Macroscopically, the bottom yellow layer is a 8 cm thick rubbly zone of alternating silt and clay layers with occasional stringers of fine silt. On a microscopic scale, this layer is characterized by alternating zones of clay clasts that are covered by thin clay... 

The black layer is a 3.5 cm thick, black to very dark gray clayey silt with weak lamination. The most notable feature of this layer is the abundance of euhedral, starburst shaped Mn oxide minerals that average 0.5 to 0.1 mm in diameter (Figs. 4E and 4F). The detrital component of this unit is dominated by quartz silt. The upper contact of this unit is slightly eroded and marked by clay clasts and a transition to red laminated clay. 

The red layer is a 1 cm thick zone of red laminated clays and numerous clay clasts. In this interval are at least five sets of clay clasts overlain by clay laminae and capped by an iron-rich coating. The red clay laminae drape over individual clasts and are also deformed beneath clasts (Fig. 4G). Certain clast layers are predominantly composed of massive silty gray clay. Clasts are relatively large and elongated (up to 3 mm long x 1 mm wide) compared to other units. The upper contact with the upper orange layer is sharp and planar. 

The thin, clay and iron-rich red layer caps a transition to a calmer water depositional environment. The red layer is predominantly composed of quiet water clay laminae, but at least five separate episodes of turbulent flow introduced clay rip-up clasts into the system. The red layer marks the last occurrence of remobilized clay clasts. These were in turn covered by laminae deposited during the quieter phase of flow. The high iron and kaolinite clay contents suggest an influx of weathered soil sediment. The iron enriched caps on fining upward laminae implies that an extraordinary amount of iron was allowed to settle or precipitate out in quiet water. 

Well Depth (mRKB) Formation Quartz clasts K-feldspar clasts Plagio- clase clasts Mica clasts Heavy minerals Carbon- ate fossils Plant frag- ments Clay clasts Clay matrix Pyrite cement Authi- genic kaolinite Authi- genic illite Calcite cement Siderite cement Dolomite cement Quartz cement Porosity Grain size (mm)... 

Together with calcite, other diagenetic features of the Namorado Sandstone include dolomite, opal, kaolinite, K-feldspar overgrowths, chlorite, albite, anatase, barite, pyrite and pseudomatrix generated by squeezing of clay clasts. The whole diagenetic evolution is detailed by Carvalho (1990), Abreu et al. (1992) and Carvalho et al. (1995). 

Structural clay is of diagenetic origin, i.e., formed within the sandstcuie framework as a deposit of clay clasts. 

The top orange layer is the result of more flooded conditions. Storm events introduced sediments that became multiple fining upward silt-clay laminae. Water must have been present in the cave to promote differential sediment settling, while also preventing the creation of clay rip-up clasts. The laminae vary in thickness, and may represent an ancient record of storm/flooding events in this watershed during the Quaternary. Like the red layer, increased kaolinite and ferrihydrite indicate a more intensely weathered or more deeply eroded sediment source. 

The most widespread fill material is reddish brown (2.5 YR 4/4, 5 YR 4/4) loam with a minor admixture of relatively large oolitic bauxite pebbles (derived from the Late Triassic - Camian - beds) and coarse clasts of black chert. Pilot X-ray diffraction analysis revealed mostly muscovite/illite, plus mixed-layer clay minerals of illite/montmorillonite type, chlorite plus mixed-layer clay minerals of chlorite/montmorillonite type, calcium montmorillonite, and diaspore plus gibbsite, or just traces of bauxite minerals (Misic, 2000). The mineral composition is not as uniform as might be expected, and further research, intended for application of factorial analysis, is in progress. A potential sediment source area in the present Cerkniscica River basin (Fig. 1) appears obvious at first glance, but similar outcrops of bauxite and chert do also appear at other sites that are not much more remote. 

The site from Puerto Rico, Cueva El Mora (Fig Ic), consists of remnants of a cave clay with clasts and bones of the extinct rodent Elasmodontomys, indurated and capped with flowstone. A U/Th date on the flowstone capping yielded an age of 108.9 +15.4, -13.3 ka, again placing the fossiliferous deposit most likely within MIS 5 (McFarlane et al., in press). 

The sediments in Cueva El Mora are as much as 3 m above the modern floor, and consist of clay with clasts and fossils but again with no indication of stratification. The clasts, of the order of 1-2 cm in diameter, are rounded to sub-rounded cobbles. The clay is also is full of holes, and contains turtle plastron fragments. The sediment beside the wall is somewhat indurated and has remained in place while the rest of the floor has since been removed. Stream flow is presumed to occur during extreme events such as hurricanes but no flow is observed in the cave today and no stream deposits have been emplaced since this Last Interglacial remnant. 

Paleosols. Paleosols formed before 2.2 Ga tend to contain iron silicates, rather than siderite-iron carbonate. Rye et al. (1995) used carbonate-silicate mineral equilibria in the 2.75 Ga Mount Roe paleosol to estimate the partitioning of COa between soil and air in the late Archaean. Their calculations suggest a maximum partial pressure of 10 14 (0.04) atmospheres COa in the late Archaean atmosphere, significantly lower than the estimate of Lowe and Tice (2004) based on nahcolite. A lower limit for atmospheric COa comes from the study of siderite-clay mineral equilibria for weathering rinds on clasts in river gravels from the 3.2 Ga Moodies Group of the Barberton greenstone belt, in south Africa (Hessler et al., 2004). In this study the minimum partial pressure for atmospheric COa at 3.2 Ga was calculated to be 2.5 X 10 3 bars at 25°C. 

A very important factor is the location and distribution of clay particles within the rock. The total clay content or clay type are not dependable indicators of the susceptibility of the rock to damage. The location of clays and the clay growth form tend to control the degree of damage susceptibility. A rock in which most of the clay is confined to shale streaks or mud rip up clasts is likely to be less susceptible to damage compared with the rock in which clay is present in the pore lining form. 

The Mizunami Group is unconformably overlain by Pliocene to Pleistocene rocks of the Seto Group (5-0.7 Ma). These rocks consist of poorly consolidated fluvial sediments containing clays, silts and conglomerates with rhyolite or chert clasts. 

The succession of versicolor marly mudstones (from V to M) has a high proportion of clay minerals and a low proportion (10-5%) of calcium carbonates (calcite) the sandy structure (C) shows a high proportion of silica (quartz), a variable amount of soft clasts (clays and carbonates) and a low proportion of matrix and cement (<20%) and the sulphated inteiwals (SI and S2) have a variable amount of carbonate ( 20%). All the materials of the outcrop contain a significant quantity of gypsum that acquires a wide diversity of single or twin habits and colours of this mineral. 

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