SAMPLING AND LABORATORY METHODS

As part of the evaluation of erosively-based MTDs, thin-sections were made from Type II (chaotic mudstone) rock samples collected in the field and analysed under a light-transmitting microscope. Representative whole-rock samples of SGF deposits were carefully collected from consolidated outcrops using a geological hammer and a chisel. It was ensured that samples were collected from individual deposits (MTDs), not from amalgamated deposits (MTCs).

A pilot study was initially undertaken to determine an effective method for sampling mud-rich and typically well-weathered deposits. Whole-rock samples from Type II MTDs were collected at outcrop and were documented as either ‘erosive’

(where visual or evident erosion is frozen in situ at outcrop, such as blocks of sandstone incorporated into the base of debris-flow deposits, or where an irregular base is observed to incise into the underlying substrate), or ‘non-erosive’ (where erosion is not observed at outcrop, such as the base of the showing tabular basal surfaces with no interaction of the underlying substrate). In the pilot study, single rock samples were taken from 29 chaotic mudstones. Of this sample population, 22 MTDs showed basal erosion at outcrop and 9 appeared as non-erosive. From these, four outcrops were selected to undertake a more rigorous sampling technique, collecting samples at equal increments from base-to-top of each deposit. Multiple rock samples were taken from 4 chaotic mudstones, which were sampled systematically from the base-to-top in equal vertical increments (DF-1, DF-2, DF-3 and DF-4) (Figure 3.6)

Figure 3.6. (A) DF-2 White dashed line indicates base of Type IIa MTD eroding into muddy substrate and potentially eroding into sandstones. White line is ~ 50 m in length (Locality 24) (B) DF-3 White dashed line indicates base of Type IIa MTD showing non-erosive planar base.

Compass for scale (10 cm) (Locality 4) (C) DF-4 -Type IIa MTD showing irregular base. White dashed line indicates reference point to depth that MTD erodes to. Type IIa MTD does not show the ‘plucking’ process at this outcrop. Pencil is for scale (15 cm) (Locality 6).

Locality 23 shows sandstone blocks ‘plucked’ into the base of Type IIa MTDs (Chapter 6, Figure 6.3), sampled as ‘DF-1’. Sample ‘DF-2’ (Figure 3.6-A) was taken near Locality 24, observed to erode into mudstone. Through mapping, it also appeared to erode into sandstone, however this is not observed directly at outcrop. Sample DF-3 was taken at Locality 4 (Figure DF-3.6-B), and appeared as non-erosive at outcrop.

Sample DF-4 was taken at Locality 6 (Figure 3.6-C), where although blocks of sandstone were not incorporated into the base, the Type IIa MTD shows an irregular base, and therefore was interpreted as likely to be erosive.

Sandy SGF deposits (i.e., turbidites) were also sampled and used as a reference for sand-rich deposits to be able to differentiate between the composition debris-flow deposits that might have been eroded and incorporated into debris flows.

Sandy SGF deposits of different grain-sizes were selected at outcrop to account for grain-size differences.

In the laboratory, sieve analyses of the rock samples could not be undertaken, as the digestion process required for ancient lithified sediments would eliminate the calcite cement, calcite grains, calcareous muds and carbonate grains. The crushing process required for sieve analysis would also grind the rock producing smaller grain-sizes, resulting in erroneous data. Therefore, samples were prepared for thin-section at the Open University. All debrite samples required resin impregnation due to the muddy nature of the samples. Thin sections were analysed under a transmitted-light microscope using standard point-counting methods to determine the variation between erosively-based and non erosively-based deposits, and also to compare any differences in composition between debrites and sandy SGF deposits. Standard point-count techniques were undertaken using methods from Middleton et al. (1985), with every grain that fell under a grid-point measured. Thin-sections were also photographed and described. Between 500 and 1,000 counted points were taken for each sample to ensure statistically valid results.

Point-count analysis was undertaken to identify the bulk composition of grains and the relative abundance of grains versus matrix found in debrites. Polycrystalline and monocrystalline quartz, carbonate grains, lithic fragments (sedimentary and metamorphic), feldspar (P and K), nummulites, coral fragments, opaque minerals, mica and calcite were identified and their abundance calculated. Polycrystalline quartz grains are likely to be metamorphic fragments, but in this study, they are grouped with monocrystalline quartz (‘total quartz’), as they are the same mineral composition. Matrix was identified as silt- and mud-grade elements, below 0.06 mm (Figure 3.7).

Figure 3.7. (A) Photomicrograph of E-A1 (B) Photomicrograph of E-D1 (C) Photomicrograph of E-F1 (D) Photomicrograph of E-I1. (Csc) Carbonate grain (sparry cement); (Cmm) Carbonate grain (micritic matrix); (PQz) Polycrystalline quartz; (Qz) Quartz; (F) Feldspar; (Ca) Calcite;

(M) Mica; (Sl) Sedimentary lithic fragment; (Ml) Metamorphic lithic fragment; (Mx) Matrix;

(O) Opaque minerals. Images are 4.4 mm wide.

Grain-size analyses were undertaken to determine variation in grain-sizes throughout all deposits sampled. To minimise misidentification between silt and clay grains, 0.06 mm was the lowest sand fraction recorded. Grain-sizes were recorded as:

 Fine pebble gravel (4 - 6 mm) – maximum grain size;

 Very-coarse sand (1 - 2 mm);

 Coarse sand (0.5 - 1 mm);

 Medium sand (0.25 - 0.5 mm);

 Fine sand (0.125 - 0.25 mm), and;

 Very fine sand (0.06 - 0.125 mm), minimum grain size.

The sampling technique undertaken for the four MTDs sampled from top-to-base provided a meaningful dataset. This included sampling greater than three samples per deposit and ensuring samples were taken systematically from top-to-base at equal increments. Based on this evaluation, a further 129 debrite samples were collected and prepared as thin sections from 18 other debris-flow deposits in the Ainsa Basin, culminating a sample population of 22 deposits (including the 4 MTDs that formed part of the pilot study). A full sample catalogue is provided in Table 3.2.

Table 3.2. Sample catalogue of samples taken from Type II MTDs (samples from the pilot study are included).

Composition and grain-size analyses were carried out using standard point counting, with methods described previously, to determine vertical grading and compositional differences between each deposit. Eighteen sandy SGF deposits were sampled at outcrop to compare the composition between mud-rich debrites, however, the vertical sampling technique was not undertaken for the sandy SGF deposits. Of

the sandy SGF deposits samples taken, two appeared as muddy sands at outcrop, with the aim to observe compositional similarities or differences between muddy-sands (sandy SGF deposits) and sandy-muds (debrites). Results from these analyses (the pilot study and full analytical study) are presented in Chapter 7.