Distinguishing between discrete MTDs within MTCs at outcrop can be difficult as it is not always obvious whether compositional or fabric changes are present due to varying internal dynamics of individual flows, or whether individual MTDs have unique sedimentary properties (cf. figure 10 of Major [1997]. In the Major (1997) study, amalgamation surfaces of successive debrites proved difficult or impossible to observe without an intervening yellow sand layer). In the field, if thin, graded- to non-graded laterally continuous sandstone beds were not observed between deposits, then MTD facies attributes were used as key indicators to suggest vertically stacked deposits were likely deposited from discrete events. The following MTD facies attributes were identified:
(1) vertical burrows at the top of a deposit (to suggest a break in time between deposition of MTDs);
(2) truncated basal surfaces (inferred as indicating at least some consolidation of the lower deposit suggesting a break in time between deposition of MTDs);
(3) ‘channelised’ deposits’ (e.g., MTDs showing channel-like basal geometry, as documented in Chapter 6);
(4) topography of lower MTDs within an MTC;
(5) different fabric properties (including graded fabric within Type IIa, b and III MTDs, varying sand concentrations within the fabric of Type IIa MTDs and also colour variations), and;
(6) different pebble concentrations in Type IIa and III MTDs.
Examples of discrete MTDs documented within MTCs at outcrop are shown in Figure 4.9.
Figure 4.9. Field observations that allow the recognition of multiple MTDs within an MTC.
White dashed lines indicate discrete MTDs (Locality 63) (A) Channelised sandstone element, separating MTDs within MTC (Locality 63) Tape measure for scale (30 cm) (B) Stacked Type IIa MTDs showing sharp break between deposits, differentiated by distinct colour difference, (Locality 29) compass-clinometre for scale (10 cm) (C) Type IIa MTD showing erosion surface above Type Ia MTD (Locality 40). Yellow notebook for scale (20 cm).
When producing sedimentary logs in the field, where possible, MTCs were broken down into discrete depositional events (e.g., Figure 4.10, Localities 31 and 44).
Using the sedimentary logs recorded in the field, the potential genetic relationships between successive events within an MTC were evaluated (i.e., genetically related beds sensu stricto hybrid event beds, Haughton et al. [2003, 2009]). Only MTCs comprised of two discrete deposits (MTDs) were used to define facies pairs and the stacking patterns of MTDs (Figure 4.11).
Figure 4.10. (A) At 11 m, MTC comprising lower Type Ia deposited below Type IIa MTD. M-II Fan (Locality 31) (B) MTCs comprising Type IIa and Type Ia MTDs, with pebble layers. B-VI Fan (Locality 44). Scales of sedimentary logs are in metres.
Figure 4.11. Stacking patterns of vertically stacked Facies Pairs logged in the Ainsa Basin that forms 3 Facies Groups (A, B and C). Facies Numbers 1 to 15B denotes number relating to stacking pattern.
Facies Group A represent MTDs documented as discrete event beds, such as they appear as isolated chaotic deposits within non-deformed sandstones and/or mudstones packages (Numbers 1 to 7). Facies Groups B and C represent MTCs (stacked MTDs), where thirteen stacking patterns are identified (Numbers 8 to 15B).
Facies Group B shows Facies Pairs comprised of the same MTDs (i.e., vertically stacked Type IIa deposits). Facies Group C shows Facies Pairs comprised of different MTDs. For example, Facies Pair ‘11A’ represents a Type Ia MTD deposited above a Type IIa MTD, and ‘11B’ shows the reverse stacking pattern.
A summary of the MTD facies attributes, with total number of occurrences are identified between each facies pair (Table 4.1).
Facies Group Facies pairs Facies number Number of occurances found in sedimentary log Facies associations In some cases, burrows at top lower sediment slump/slide
Stacked Type IIa MTDs (MTC) 9 17
In some cases, vertical grading of sand (higher sand concentration at the base) In some cases, debrites can show laminated top
In some cases, concentration of pebbles at base of debrite In some cases, colour variation between debrites In some cases, upper debrite shows erosion into lower debrite
In some cases, differing concentration and sizes of pebbles between debrites In some cases, different fabric properties (i.e. well-mixed or patchy) between debrite In some cases, burrows at top of lower debrite
In some cases, different sand concentrations between debrites
Stacked Type III MTDs (MTC) 10 4
In some cases, pebbly sandstones show normal grading
In some cases, pebbly sandstones show clast-supported base/matrix-supported top Stacked pebbly sandstones typically show different sized pebbles
11A 20
Multiphase slump/debrites (commonly found in Guaso System MTCs) In some cases, burrows at top of lower debrite
In some cases, lower debrite shows topography
11B 11
Multiphase slump/debrites (commonly found in Guaso System MTCs) In somes cases, upper debrite truncates lower sediment slump/slide In some cases, burrows at top of lower sediment slump/slide In some cases, lower sediment slump/slump shows topography
12A 4 In some cases, upper pebbly sandstone shows erosive base or infills topography 12B 6 In some cases, upper debrite erodes into lower sandstone
In some cases, underlying pebbly sandstone can produces topography 13A 3 Upper conglomerate is typically channelised
13B 19 Upper debrite can be erosive incorporating underlying pebbles
Type Ia and III MTDs (MTC) 14A 5 In some cases, underlying pebbly sandstone produces topography
Type Ic and IV MTDs (MTC) 15A 4 Base of upper carbonate is typically brecciated
Lower debrite shows angular bioclastic clasts beneath carbonate rafts Type Ia and Iia MTDs (MTC)
Type IIa and III MTDs (MTC)
Type IIa and Iib MTDs (MTC)
Occur as discrete chaotic deposits within non-deformed sandstones/mudstones
FACIES PAIR B -stacked MTDs of the same faciesFACIES PAIR C -stacked MTDs of different facies (A and B facies pairs show opposite stacking order)FACIES PAIR A- Discrete MTDs
Table 4.1. Summary of MTD facies attributes found between vertically stacked discrete MTDs.
Facies attributes documented between MTDs are noted ‘in some cases’ as not all outcrops show all the described characteristics.
To consider the genetic relationship between stacking patterns of MTCs comprised of different deposits (Facies Group C), a probability value (p-value) was calculated using a chi-test to compare observed data with the expected distribution.
Facies Group A occur as isolated deposits and Facies Group B comprises MTDs of the same facies, therefore it is not possible to undertake a similar chi-test analysis. To achieve this for Facies Group C, the following hypotheses were considered:
Null Hypothesis: There is no relationship between the stacking patterns of Facies
Pairs.
Alternative Hypothesis: There is a relationship between the stacking patterns of Facies Pairs.
In these hypotheses, facies ‘relationships’ relate to the random or predictable pattern of stacked MTD facies. If the chi-test determined a P-value of < 0.5, the Null Hypothesis is rejected. Results are shown in Table 4.2.
Facies Facies Pair Actual range (observed number) Expected range (Total/number of subjects) Chi-test (result from Excel) Result
Fa ci es Gr o u p C
Type Ia and IIa
11A 20
15.5 0.10 < 50% reject null hypothesis
11B 11
11 0.0006 < 50% reject null hypothesis
13B 19
Table 4.2. Results of chi-test facies analysis for Facies Pairs that show different MTDs in stacked succession.
The p-value for stacking patterns of Facies Pair 12 (A and B) was > 0.5, therefore the Null Hypothesis is accepted, i.e., there is no significant relationship between the stacking patterns between Type II and Type III MTDs. The p-value of stacking patterns of Facies Pairs 11, 13, 14 and 15 is < 0.5, therefore the Null Hypothesis is rejected, consequently suggesting a potential relationship between the stacking patterns of the following Facies Pairs:
(1) Facies Pairs 11a, b: Type Ia deposits immediately above Type IIa deposits (Facies 11A) are documented as more abundant.
(2) Facies Pairs 13a, b: Type IIa deposits immediately above Type IIb deposits Haughton et al., 2003, 2009), or potentially from similar source areas.