Infiltrated clays

Chlorite replaces kaolinite, clay pseudomatrix, infiltrated clays, micas and heavy minerals. It occurs as rims composed of platelets oriented perpendicularly to grain surfaces. The rims were formed by replacing infiltrated smectitic clay coatings, which were originally oriented tangentially to grain surfaces (Fig. 14A) (see Moraes De Ros, 1990). These infiltrated clays were presumably introduced into the vadose zone of alluvial continental sediments under semi-arid conditions by episodic floods (Walker et al., 1978 Moraes De Ros,... 

Typically, mechanically infiltrated clays are originally detrital smectites formed under semi-arid weathering conditions (see Keller, 1970 Walker et al., 1978). This is evidenced by the dominance of smectitic clays in the mudstone samples and in the mud intraclasts. Infiltrated coatings and derived chloritized rims are conspicuous, particularly in medium-grained sheet-flood sandstones (up to 2.7 vol%). 

Hematite occurs sparsely in the fine-grained flood-plain sediments, as tiny pigments that are either evenly distributed in the sediment or closely associated with infiltrated clay coatings around framework grains, and as alteration products of detrital Fe-bearing minerals such as Fe-Ti oxides. 

Although both cores display floodplain mudstones and sheet-flood sandstones, the Middle Lunde samples in 34/7-A-3H are dominated by fine- to medium-grained sheet-flood sandstones. The elevated initial porosity and permeability of these sandstones compared with the mudstones, siltstones and very fine to fine-grained sandstones has perhaps allowed larger amounts of mechanically infiltrated clays, which are preserved as smectitic coatings and/or transformed into chloritic or C/S and I/S rims in the sandstones. 

Sandstones with potentially better porosity preservation are characterized by (i) coarser grain size and better sorting (ii) lower tendency to host extensive eogenetic carbonate cement than the finer sediments, which are more represented by well 34/4-1 samples and (iii) chlorite rims evolved from the infiltrated clay coatings, which are more abundant in coarse-grained sands which inhibited precipitation of pore-occluding quartz and carbonate cements. 

Moraes M.A.S. De Ros, L.F. (1990) Infiltrated clays in fluvial Jurassic sandstones of Reconcavo Basin, northeastern Brazil. J. sediment. Petrol, 60, 809-819. 

The semi-arid climatic conditions promoted clay infiltration. The greater abundance of infiltrated clays in the proximal than in the other domains (Table 1) is attributed to more frequent flooding during episodic rainfall, and infiltration of suspended mud into the coarser-grained, more permeable proximal sands. The infiltrated clays were originally smectitic, derived from the chemical weathering of source rocks under the semi-arid climatic conditions (Keller, 1970 Curtis, 1990). 

---