3D mechanical stratigraphy
3D mechanical stratigraphy of a deformed multi-layer: Linking sedimentary
architecture and strain partitioning
Stratigraphic inﬂuence on structural style and strain distribution in deformed sedimentary sequences is well established, in models of 2D mechanical stratigraphy. In this study we attempt to reﬁne existing models of stratigraphic-structure interaction by examining outcrop scale 3D variations in sedimentary architecture and the eﬀects on subsequent deformation. At Monkstone Point, Pembrokeshire, SW Wales, digital mapping and virtual
scanline data from a high resolution virtual outcrop have been combined with ﬁeld observations, sedimentary logs and thin section analysis. Results show that signiﬁcant variation in strain partitioning is controlled by changes, at a scale of tens of metres, in sedimentary architecture within Upper Carboniferous ﬂuvio-deltaic deposits. Coupled vs uncoupled deformation of the sequence is deﬁned by the composition and lateral continuity of mechanical units and unit interfaces. Where the sedimentary sequence is characterized by gradational changes in composition and grain size, we ﬁnd that deformation structures are best characterized by patterns of distributed strain. In contrast, distinct compositional changes vertically and in laterally equivalent deposits results in highly partitioned deformation and strain. The mechanical stratigraphy of the study area is inherently 3D in nature, due to lateral and vertical compositional variability. Consideration should be given to 3D variations in mechanical stratigraphy, such as those outlined here, when predicting subsurface deformation in multi-layers.
While the concept of mechanical stratigraphy to explain and predict structural behaviour is a powerful tool, these models are generally applied to continuous sedimentary sequences based on 2D sections through the stratigraphy. Mechanical layer and interface properties of rock units are commonly greatly simpliﬁed and presumed to be internally homogenous and laterally continuous. Only rarely is the presence of heterogeneities internal to units or the superposition of structure on outcrop-scale sedimentary features acknowledged when considering stratigraphic-structure interactions, albeit in 2D. Some studies have addressed this interaction in 3D, using seismic data, who describe the development of multiple thrust faults in diﬀerent sedimentary layers, that later link into single through-going thrusts.
Our example is from the contractional setting of Monkstone Point in Pembrokeshire, SW Wales (Fig. 1), which exposes a layered sequence of deformed clastic sediments. A genetic-link is predicted between the 3D stratigraphic architecture and the degree of strain partitioning within this multi-layer succession. Using sedimentary/structural logging, Structure from Motion photogrammetry, compositional analysis of samples and structural measurements we provide a detail analysis of the outcrop and document the extent and localisation of partitioned strain during contractional deformation.
Monkstone Point lies within the South Wales Lower Coal Measures Formation (312–313 MA), of the Upper Carboniferous (Westphalian), which is dominated by coal-bearing mudstones and siltstones, with minor sandstones present in the lower part of the succession (Jenkins,
1962; Williams, 1968; George, 1982; Waters et al., 2009). This sequence comprises part of the post-rift stratigraphy of SW Wales, post-dating units associated with Silurian and Devonian extension. The South Wales Lower Coal Measures Formation is interpreted as ﬂuvio-deltaic in origin (George, 2008), and is characterized locally by distributary channels, coastal plain and delta slope deposits, and shallow marine sequences (Powell, 1989; George, 2000). Channel bodies within the lower part of the succession are characterized by erosive bases and are commonly cross-bedded. Minor seat-earths, overbank deposits and thin coal beds are distributed throughout the succession, and are often constituents of channel-base lag deposit material.
The northern limit of Variscan deformation in the United Kingdom falls in SW Wales, and dominates the deformation structures in the South Wales Lower Coal Measures Formation. Regionally, the structural trend is WNW – ESE (Fig. 1c), marking NNW-directed Variscan shortening of up to 67% (Frodsham and Gayer, 1997). Vergence of folds and thrust sequences is generally to the NNW (Fig. 2), though deformation is commonly accommodated by back-thrusts with local folds verging to the SSE
Monkstone Point lies on Pembrokeshire’s east coast (Fig. 1c) and is an exposed anticline in the South Wales Lower Coal Measures Formation. Outcrop morphology provides three natural, N-S oriented cliﬀ sections, spaced ∼100 m apart (Fig. 1b), allowing examination of lateral and vertical variations in sedimentary facies documented in 65 m of logged section. Natural Gamma Ray (NGR) data was collected for each of the logged sections with a Radiation Solutions RS-230 Handheld Radiation Detector. In-ﬁeld measurements were performed, where possible, on unfractured, continuous rock faces, with the tool held on the rock face for 180 s, providing estimates of %K and ppm values for U & Th. NGR data is presented here as total counts per second (cps). This technique was employed to supplement logging by objectively quantifying changes in composition through the sedimentary succession.
3.1. Mudstone (Unit 1)
A ﬁne-grained mudstone, up to 2.4 m thick, is the base unit of the three logged sections (Figs. 3). Grain size ranges from ∼1-4 μm, and grain composition is dominated by quartz grains, micas and feldspar. High NGR values of 97–178 cps, and correspondingly low Si bulk
composition (16–27%) suggest this unit contains a signiﬁcant amount of ferro-silicates and muddy material, supported by thin section ob-
servations (Fig. 3f). Unit 1 was likely deposited in a delta-plain environment, associated with lagoonal or estuarine facies (Bluck and Kelling, 1963). Depositional or erosive contacts of overlying Units 2, 3 and 4 suggest this is the stratigraphically oldest unit at Monkstone Point.
3.2. Fine interbeds (Unit 2)
Directly above the mudstone (Unit 1) is a laterally discontinuous, chaotic package of mudstones, thin coal horizons with sandstone in- terbeds. This package, 0.2–1.1 m thick, contains numerous intraformational mudstone lag deposits, plant material, and erosive contacts. Variability of grain size (1–15 μm) and NGR counts (94–124 cps), reﬂect the heterogeneity in this package manifest as rapid transitions in grain size and composition vertically. As with Unit 1, Unit 2 probably represents lagoon or estuary deposits, with thin, discontinuous sandstones, coal horizons and terrestrial material re- presenting local, laterally discontinuous variations in depositional environment.
3.3. Basal gravel lag (Unit 3)
Unit 3, a gravel lag deposit, incises downwards into Units 1 and 2 and forms the erosive base to the overlying sandstone channel sequence (Units 4 and 5). Clasts within this unit are comprised of quartz and ironstone pebbles mixed with coal fragments, rare plant material and carbonaceous shale, interpreted as locally derived (Bluck and Kelling, 1963), with a grain size distribution from silt to pebble sized grains. This unit is laterally continuous across the outcrop and has a thickness range of 0.4–2.2 m. The erosive nature of the unit’s lower bounding surface and a lack of internal sedimentary structure are suggestive of chaotic, rapid deposition. This gravel lag body records widespread erosion and a marked increase in grain size across the outcrop and thus probably records incision of a major channel in the sedimentary sequence (Miall, 1985).
3.4. Sandstone (Unit 4)
This sandstone interval comprises the thickest vertical component (up to 18.3 m) of the sedimentary sequence at Monkstone and lies conformably on, or incises downwards into, Unit 3. In general, this part of the sequence ﬁnes upwards, contains an erosive base overlain by ﬁner, siltstone beds and has a composition dominated by quartz grains (> 80%), with a grain size range of 5–25 μm . XRF measure- ments of Si content within this unit were measured from 30 samples (24 of which were sampled in log B). The silica content range (15–34.5%) in Unit 4 reﬂects compositional variability within this unit.
Similarly, Gamma Ray counts, from 27 measurement sites in Unit 4, display a range of values (63–146 cps). NGR and XRF data (Fig. 3b and c), along with ﬁeld logs (Fig. 4), also record these lateral and vertical variations in the composition of Unit 4.
3.5. Upper siltstones (Unit 5)
Unit 5 is a ﬁning-upward siltstone, a continuation of the underlying Monkstone Sandstone, from which the change to siltstone is gradational. Measurements from this unit record a small grain size distribution (3–5 μm) and low variability in Silica content and Gamma Ray measurements (16.8–28.7% and 82–109 cps respectively). Poor exposure of this upper part of the sequence does not allow an estimation of total thickness, but a recorded minimum of 4.6 m. The absence of erosive bed contacts within this unit and the transition to greater homogeneity in the sequence may be suggestive of increasing marine inﬂuence at a delta-front position, in contrast to the distributary setting associated with Unit 3 and parts of Unit 4.
An abundance of meso-scale structures at Monkstone Point allows detailed assessment of the layer-by-layer structural style, its evolution and consequently characterisation of the mechanical stratigraphy. This section aims to evaluate the observed patterns of deformation internal to each unit, at unit interfaces, and through the exposed sequence. To assess lateral variations in sedimentary architecture and impacts on the assigned mechanical stratigraphy, virtual scanlines were used to record the frequency and spacing of thrust faults or fault segments (discrete slip surfaces) through units in each of the cross-sections. This was carried out by digital mapping of fault segments on the virtual outcrop along parallel scanlines through Units 1–4. Measured fault oﬀsets and approximate positions of fault-scanline intersections is presented in We combine the scanline data with sedimentary logs, cross-sections and ﬁeld observations to assess the link between sedimentary architecture and observed partitioning of strain.
1. Multi-scale outcrop data of a deformed multi-layer at Monkstone Point, SW Wales highlights complex patterns of strain partitioning, and shows that there are a number of factors that may control spatial and temporal variations in how strain is distributed.
2. Variations in internal architecture and composition of the sedimentary sequence, controlled by depositional environment, impacts on where strain is localised. In each of the serial cross-sections, each mechanical unit records a distinct structural style, deﬁned by the composition and internal structure of that unit, and the nature of the interfaces between units. Structural style thus changes vertically through each of the cross-sections.
3. In addition to changes in structural style through the vertical succession, each of the closely spaced (< 100 m), serial cross-sections records signiﬁcant diﬀerences in structural style in laterallyequivalent successions. Lateral changes within units, in composition (e.g. relative abundances of ﬁne-grained material), internal structure (e.g. the occurrence of cross-bedding) and architecture (e.g. lateral extent of ﬁne-grained horizons) impacts on where deformation is localised.
4. Temporal evolution of the mechanical stratigraphy records a transition from early, unit-partitioned meso-scale deformation to mid-
stage distributed strain to late-stage, highly localised strain along discrete fault planes, which deform the entire sequence.
5. This study shows how mechanical stratigraphy varies not only vertically through a succession, but both laterally and through progressive deformation. Additional studies such as this may reﬁne existing 2D structural models which assume internally homogenous layer cake stratigraphy.
By Adam J. Cawood, Clare E. Bond