GEOLOGICAL JOURNAL, VOL. 31, 189-195 (1996)

Significance of infiltrated clays within the Lower Permian Yellow Sands of north-east England

GREGORY D. PRICE' Postgraduate Research Institute for Sedirnentology, The Uriiversity of Reading, PO Box 222

Whiteknights, Reading RG6 2AB. UK

The Permian Yellow Sands are preserved as nine aeolian ridges in north-east England. The sands are soft and friable and locally contain large amounts of interstitial detrital clays, which are interpreted as the product of early mechanical clay infiltration. Features within the sands considered to be due to mechanical clay infiltration include grain coatings, meniscus and geopetal fabrics. The infiltrated clays mask crystal nucleation sites on grain surfaces and often amalgamate into lenses or form continuous clay-rich layers. Calcite and quartz cement growth has consequently been limited and often confined to the coarser, generally clay-free horizons. The meniscus and geopetal fabrics were formed early during accretion of the dunes and helped to bind and stabilize the sediment, which restricted reworking during the Zechstein transgression to the uppermost few decimeters.

KEY WORDS infiltrated clays; Permian Yellow Sands; aeolian sandstones

1. INTRODUCTION

The mechanical infiltration of clay was first documented as a product of early diagenesis by Walker and Crone (1974), Walker (1976) and Walker et al. (1978). A number of studies (e.g. Tsoar and Moller 1986; Pye and Tsoar 1987) have also documented the occurrence of clay and silt infiltration during the accretion of modern desert dunes. More recently the main variables that influence the amount of clay infiltration and criteria for the positive identification of infiltrated clays have been described (e.g. Matlack et al. 1989; Moraes and de Ros 1990). Features attributed to be the product of early mechanical clay infiltration have been documented for a number of ancient sandstone sequences (e.g. Kessler 1978; Molenaar 1986; Moraes and de Ros 1990). Molenaar (1986) and Moraes and de Ros (1990) have also examined how mechanically infiltrated clay can affect the porosity and permeability distribution pattern.

The aim of this paper is to describe the occurrence of infiltrated clays within the Permian Yellow Sands and to show how these features may have affected subsequent diagenesis and the deformation of the dunes by the Zechstein transgression. Detailed sampling was carried out on three principle sections of the sands exposed at North Hylton, Ferryhill and Old Quarrington Quarry (Figure 1). These sites offered high quality exposures, and access for sampling all parts of the sequence was possible.

2. GEOLOGICAL SETTING

The Permian Yellow Sands are believed to represent part of a large erg on the western margin of the southern Permian Basin and are equivalent in age to the uppermost part of the Rotliegend sandstone (Glennie 1984). The sands crop out in a thin belt from South Shields to Ferryhill (Figure 1) and are preserved as nine mounds elongated north-east to south-west (Smith and Francis 1967; Steele 1983), which

*Present address: Palaeoecology Centre, School of Geosciences, The Queen's University of Belfast, Belfast BT7 INN, UK.

CCC 0072-1050/96/020189-07 Q 1996 by John Wiley & Sons, Ltd.

Received 18 November 1994 Revised 30 September 1995

190 G. I). PRICE

Figure I . Outcrop map ofthe Pcrrnian Yellow Sands showing sample localities. Modified from Krinsley and Smith (1981)

arc thought to reprcscnt linear dunes largely buried intact. They have been interprctcd as aeolian by many workers, including Smith and Francis (l967), Smith (l970), Glennie (1984). Yardley (1984), Clemmcnson (1989) and Chrintz and Clemmenson (1993), principally on the cvidcnce of the pattcrn and scale of cross- stratification and the absence of marine fossils. The Yellow Sands, in this paper, have been intcrpreted to be of aeolian origin based on the widcspread recognition of invcrsely graded strata at outcrop. This thinly laminated type of deposit is thought to have formed by climbing wind ripples (Hunter 1977) and is one of the most diagnostic features of an aeolian deposit (Kocurck and Dott 1981).

The sands arc composed mainly of high-angle cross-bedded dcposits (dune centrc), low-angle cross- bedded (dune plinth) deposits and flat-bedded (intcrdune) sands (see Clemmenson 1989). A thin, generally structureless, unit lics uppermost in the sands and is thought to bc the product of reworking of the sands during the Zechstein transgrcssion (Smith 1970) (Figure 2). Syn-sedimentary folds are present within this unit at Ferryhill and Glennie and Buller (1983) observc vaguely laminated horizons which may represent local occurrcnccs of beach or nearshore sands. The Late Pcrmian Marl Slatc (equivalent to the Kupfer- schiefer) overlies the sands and was dcposited during the Zechstein transgression, which began a prolongcd period of marine conditions. The petrology of thc Yellow Sands has been documented by Pryor (1971), who considered thc sands to be typical of a shallow marine deposit, and by Krinslcy and Smith (1981), who accepted the aeolian model.

3. ANALYTICAL PROCEDURES

The Yellow Sands wcrc examined using standard optical petrography and by scanning electron microscopy (SEM) with a LINK SYSTEMS energy-dispersive X-ray analysis (EXDRA) system. Representative

INFILTRATED CLAYS, PERMIAN OF N E ENGLAND 191

1 Thinly bedded limestone and dolomite

Massive reworked horlzon

, FGiA Synsadimntary Slumps

--/ I

6

f z Calcite cemented high-angle cross

4 bedded strata

3

2

Highangle cross bedding

Prominent quam cemntad horlzon

Contact with underlying Carboniferous not seen

Figure 2. Simplified vertical lithologicdl section through the Yellow Sands exposed at Ferryhill

subsamples of the sands were ground to a powder for analysis by whole-rock X-ray diffraction (XRD) to determine the bulk mineralogy using Cu K a radiation. Clay-size (< 2 pm) fractions were also extracted from the sands by dispersion in sodium hexametaphosphate and disaggregation in an ultrasonic bath. The mixture was allowed to stand for three hours 52 minutes and the clay fraction left in suspension was then pipetted onto ceramic tiles under a vacuum suction, producing a randomly orientated layer. X R D analysis was carried out on the tiles, which were subjected to the following pre-treatments: (i) air-drying; (ii) exposure to warm ethylene glycol vapour for 12 hours; and (iii) heating in an oven for one hour at 375°C. This methodology produces semi-quantitative results.

4. PETROGRAPHY

The Yellow Sands are mineralogically mature and can be classified as sub-litharenites. Detrital quartz is the most abundant mineral present and generally constitutes between 70 and 80% of the samples. Feldspars (both plagiosclase and orthoclase) are present in varying amounts, together with rock fragments and opaque minerals (Table 1). Calcite is the most common cement and is distributed through the sediment as isolated blocky sparite crystals or as large poikilotopic plates infilling clusters of adjacent pore spaces. Locally, extensive calcite cemented layers are present and are most commonly associated with well-sorted medium- to coarse-grained grain-flow strata and with the thin reworked horizon uppermost in the sequence (Figure 2). The calcite cements are probably uplift related (Tertiary or Later) (see Lee and Harwood 1989). Quartz cements are only locally extensive. Overall, the major petrographic characteristic of the sands is the general lack of cements, which gives rise to the soft and friable nature of the sands.

Whole-rock X R D analysis also records the presence of trace amounts of anhydrite, which were not observed in thin section or by SEM analysis. Clay X R D analysis of the < 2 pm size fraction of four samples from Old Quarrington Quarry showed that the principle components of the matrix are illite (8-32%), mixed-layer clay (3-34%) and kaolinite/chlorite (38-89%). Polished thin section and SEM analysis

192 G. D. PKICE

Tablc I . Composition of the Yellow Sands determined by point-count analysis (300 counts) and grain-size analysis

I h n c centre ("0) Dune plinth (YO) Inter-dune (YO) ( 1 1 6) (n = 7) ( 1 1 = 6)

~

Iktrital grains Quart;r Orthoclase feldspar Plagioclase feldspar Rock fragments

Interstitial components Clay matrix Calcite cement Dolomite cement QuiirtI cement Kaolinite Opaque minerals

Grain size

70.1 4.5 t r

2.8

5.6 6.7 1.3 4.0 4.0 t r

1.70-2.710 2.170 (ave.)

77.3 7.1 tr

2.8

7.3 2.2 1.9 (r t r 1 . 1

I .76d (avc.) I .43 -2.270

72.2 2.6 tr

3.8

12.6 3.5 1.5 tr tr

2.1 1.36 2.074 1.844 (ave.)

n = Numbcr of samples analyscd; t r - trace amount

confirms the presencc of abundant porc-filling authigenic kaolinite forming characteristic pseudohexagonal booklets. Iron compounds and silt-size quartz grains (possibly formed by aeolian abrasion, e.g. Whalley el (11. 1987) are also an important component of the matrix. The presence of the featurcs described in the following indicates that some of the matrix must have been infiltrated into the sediment after the deposition of the sand fraction.

5. MENISCUS AND GEOPETAL FABRICS

The highest proportion of clay matrix occurs within the dune plinth and inter-dune units (Table I ) , forming extcnsivc grain coatings or amalgamating into discontinuous lenses which envelop several grains. Within some horizons these coatings are thickcr on the upper surface of clasts, forming a geopetal fabric (Figurc 3a) or concentrated at the bases of large pore spaces (Figure 3b). Coatings are largely absent at grain contacts. Sinuous ridges and bridges connecting detrital grains orientated roughly normal to the grain surface are observed by SEM. These have either a platy structure (Figure 3c) or an amorphous texture which merges indistinguishably with thc grain coating (Figure 3d). The general absencc of detrital clay coating the calcite and quartz cement crystals suggest that clay infiltration pre-dates these cements and they are therefore not rclatcd to a near-modern aquifer system. The gcopetal and mcniscus fabrics compare closely with the textures recordcd by Kesslcr (1978) from Rotliegend aeolian sandstones of the North Sca and those rccorded by Matlack et al. (1989) from Holoccnc core examplcs. Observations made by Walker (1976) and Walker ('1 ul. (1978) in Tertiary and Quaternary desert alluvium also show similar examples of fabrics thought to have formed by the infiltration of clay (dust size particles) into sediments lying above the water- table, i n the vadose zone. Infiltration is caused by the influcnt seepage of surface rain or flood waters (Walker 1976; Walkcr e / af. 1978), or due to the action of desert dew (Folk 1969). The geopetal fabrics have possibly formed where clay particles, suspended in water percolating downwards through the vadose zone, have settled out from water droplets on the upper surfaces of grains or on the bottom of pore spaces. Meniscus ridges and bridges are, according to Walker (1976), interpreted to be the product of clays settling out of suspension between films of pellicular water.

6. INFILTRATED CLAYS AND YELLOW SAND DIAGENESIS

The presence of extensive clay mineral grain coatings may have masked potential nucleation sites, preventing the growth of crystals on quartz grain surfaces, a phenomenon observed by Tillman and Alnion

INFILTRATED CLAYS. PERMIAN OF N E E N G L A N D 193

Figure 3 . (a) Photomicrograph of orientated sample showing quartz grains with iron-rich clays (arrowed) concentrated on the upper surface of grains. Scale bar 400 pm. (b) Photomicrograph of orientated sample showing iron-rich clays (arrowed) concentrated in the bottom of pore spaces and on the upper surfaces of grains. Scale bar 400 pm. (c) SEM photomicrograph of platy meniscus bridge formed between three quartz grains. Scale bar 48 pm. (d) SEM photomicrograph of meniscus bridge with an amorphous texture.

formed between two quartz grains, merging with the g,rain coating. Scale bar 50 pm

(1979) and Molenaar (1986). Additionally, Moraes and de Ros (1990) have observed that calcite cementa- tion was inhibited where mechanically infiltrated clays were most abundant. This cffect may explain why quartz and calcite cements are surprisingly sparse, within the sands, and tend to be restricted to the coarsest grain-flow horizons where the sediments are less clay-rich and pore spaces have not been occluded. Within these coarser horizons fluid flow is therefore likely to have been less restricted. Furthermore, infiltrated clay coatings on framework grains may act as nucleation sites for the precipitation of authigenic clays (Matlack ct al. 1989) and thus further occlude pore space and restrict fluid flow. The precipitation of authigenic clay minerals is, however, a potential mechanism to recrystallize or remove infiltrated clay textures.

7. DEFORMATION O F YELLOW SANDS BY ZECHSTEIN TRANSGRESSION

During a marine transgression, several factors determine whether some element of original dune topography will be preserved. These factors can be broadly classified into those related to the dune itself and those relating to the nature of the transgressive environment (Eschner and Kocurek 1988). According to Smith (1970; 1979), the presence of only a thin sequence of reworked strata and the lack of evidence indicative of contemporaneous cementation, the occurrence of inherited dune relief could only have resulted from an extremely rapid Zechstein transgression. This is supported by Glennie and Buller (1 983), who suggested that the Zechstein sea-level rise was initially a few centimetres per day and 50 m of dunes were submerged within eight months. In addition, they proposed that the dunes would have been protected from waves by a shoal system built of submerged dunes nearest to the open water.

194 G. D. PKlCE

This model of an extrcmely rapid Zechstcin transgression, with rcference to the Permian Yellow Sands of Durham and Tync and Wear, has been moulded around the inherited dune relief and the paucity of reworked strata. Howevcr, the need to invoke such a model is diminished if it can be shown that the Yellow Sands did contain a pre-Zechstein cement phase. Early ccments are effective in preserving early dune forms and can act to reduce erosion by marine processes (Eschner and Kocurek 1988). The infiltration of clays into sands, forming geopetal and meniscus fabrics, can lead to the formation of a surface crust which tends to stabilize the sand (Pye and Tsoar 1987). If such a surface crust did form it may not have been restricted to the upper surface of the dune. According to Pyc and Tsoar (1987), during the slow vertical accretion of aeolian sand bodies a stacked sequence of ccmented surface crusts would be preserved. Additionally, several studies have shown that the large-scale preservation of dune palaeotopography was due to the evaporite cementation of the dune sands (e.g. Vincelette and Chittum 1981; Fryberger et af . 1983). However, anhydritc (detected by XRD analysis) was only recorded within the sands in trace amounts. It is thercfore tentatively proposed that a stacked sequence of crusts formed by the mechanical infiltration of clay (in addition to possibly anhydrite cement) may have acted as some form of protection to the Yellow Sands during the Zechstein transgression.

8. CONCLUSIONS

The presence of geopetal and meniscus fabrics within the Permian Ycllow Sands are indicative of formation by the mechanical infiltration of clays. The infiltrated clays filled pore spaces and masked crystal nucleation sites, inhibiting the widespread growth of quartz and calcite cements within the Permian Yellow Sands. Clay coatings possibly promoted the preferential growth of authigenic clay minerals during later burial and further occluded porc space, restricting fluid flow. Thus the Permian Ycllow Sands have a soft and friable nature rcminisccnt of a much younger deposit. The geopctal and meniscus fabrics formed early, which stabilized the dunes and helped to restrict reworking of the sands to the uppermost few decimetres during the Zechstein transgression. Hence the linear dune ridge morphology can be observed today.

ACKNOWLEDGEMENTS

The work presented here was carried out with financial support from the Natural Environment Research Council. The author acknowledges the technical support provided by staff at PRIS and the help of Professor K. Pye and Professor B. W. Sellwood. I also thank the reviewers and editors of thc Geologicul Journal for making helpful suggcstions on an earlier vcrsion of this manuscript. University of Reading PRIS Contribution No 43 I .

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