ForumComment
doi: 10.1130/G37950C.1 Prelithification and synlithification tectonic foliation development in a clastic
sedimentary sequence
Simon P. Todd
Capriole Energy Consultancy LLC, 1615 Arlington Street, Houston,
Texas 77008, USA
Using observations from a single locality (Wine Strand) in the
Glashabeg Formation (Dingle Group) of the Dingle Peninsula, southwest
Ireland, Meere et al. (2016) conclude that more than 50% shortening of
these strata was accomplished during the mid Devonian Acadian event,
before lithification was complete. That the late Silurian to Emsian Dingle
Group was deformed during the Acadian is not disputed (Todd, 2015,
and references therein). Rather, it is the conclusion that the single
tectonic foliation is Acadian and was generated before lithification was
complete is refuted by several factors, not covered by Meere et al., but
which are evident from assessment of the entire area (Todd, 2015).
(1) The entire Palaeozoic of the area was affected by the end–
Carboniferous Variscan orogenic shortening. Structural observations
indicate that Old Red Sandstone (ORS) rocks demonstrably younger than
the Acadian unconformity have experienced ~14% shortening by folding
and faulting and a Rs yz strain of ~2.5 (or 40% shortening). The strain is
associated with a cleavage that is axial planar to the folds in plan and
occurs in symmetrical fans on the axial plane in cross section (Todd,
2015). In the west of the peninsula, this Variscan deformation is
evidenced by steeply dipping and cleaved Late Devonian ORS on the
north coast and inferred from the substantial uplift of the Acadian
unconformity above regional. Any interpretation of shortening in the
Dingle Group must take account of this overprint.
(2) Dingle Group folding is Acadian in age but the single tectonic
fabric is a younger overprint. The folding and faulting of the Dingle
Group clearly accomplishes more shortening (~40%) than is evidenced
by Variscan-only folds (~14%) above the Acadian unconformity (Todd,
2015). However, when strain data from the entire area are considered,
there is a smaller difference in the Rs yz strains in mudrocks above (~2.5)
and below (~2.8) the Acadian unconformity (Todd, 2015). In other
words, as much 88% of the total bulk strain in the Dingle Group is likely
due to Variscan shortening, not Acadian. The cleavage transects the axial
planes of the Dingle Group folds in both plan and section, commensurate
with Variscan overprinting of preexisting and gently tightened Acadian
folds and hence is not a product of Acadian transpression (Todd, 2015).
(3) Strain patterns are highly dependent on bedding orientation and
lithology and fabric of the sediments. At Wine Strand, and other
locations where strata are sub-horizontal, mudrocks, when pedogenic
calcrete is absent or weakly developed, are typically deformed by
constant volume flattening strain with a component of vertical stretching
(Meere and Mulchrone, 2006). In coarser lithologies, the calculated Rs yz
strains are markedly lower and instead some bulk strain occurred by
volume loss along the spaced cleavage folia (Fig. 1A). The cleavage folia
in the coarser units become more prominent on depositional fabrics like
intraclasts, gutter casts, and channel margins that have formed preferential planes of pressure solution. Where mudrocks are strongly effected
by pedogenic calcretes, the pressure solution cleavage is more prominent
and carbonate nodules and columns are flattened in it. Where beds are
more steeply dipping to sub-vertical in the Dingle Group, more flattening
and accentuation of bed-parallel depositional fabrics in coarser rocks is
observed, particularly through the rotation and packing of more
labile/ductile lithic clasts and pressure solution creating curviplanar
instead of point grain/clast contacts (Figs. 1B–1D). Pressure solution
folia are usually partitioned to the matrix, often welding clast contacts;
occasionally clasts are cut. Mica beards on quartz grains (Meere and
Mulchrone, 2006) and competent clast extension by joints/veins have
been observed (Fig. 1C). The strain incompatibility between mudrocks
and sandstones/conglomerates is more pronounced in subvertical setting,
indicated by bed-perpendicular quartz ladder veins (Meere and
Mulchrone, 2006). Hence, throughout the area there are strain features
associated with the tectonic foliation only consistent with postlithification strain. These features are less common or absent at Wine Strand, but
this is a function of the partitioned bulk strain of bed-parallel depositional
fabrics in a poorly sorted, loosely packed conglomerate fabric and
pressure solution in the domainal cleavage.
Figure 1. A: East-west vertical joint exposing the Glashabeg Formation at Wine
Strand (52.17941N, 10.38666W) where bedding/cleavage angle is ~80. The
cleavage is a spaced pressure solution fabric in conglomerate (“b”) and
sandstones (“s”). (Notebook is 20 cm long). B: Photomicrograph (3 mm wide)
of sandstone in Glashabeg Formation, Glashabeg (52.20965N, 10.35755W)
where bedding/cleavage angle is ~30. Note welded contacts, rotated and
flattened phyllite clasts and pressure solution folia marked by black iron oxide
residues. C: NNW-SSE joint face exposing a conglomerate bed in the Trabeg
Formation (lateral Dingle Group equivalent of Glashabeg Formation) at
Paddock Point (52.11421N, 10.32039W) where bedding/cleavage angle is ~10.
Cleavage is disjunctive and anastomosing partitioned to the matrix. A crosscutting ladder vein/joint, an example of a clast with strain-induced curviplanar
contacts with its neighbors (“a”) and an example of a clast with intraclast
joints/veins (“b”), and a larger clast cut by a pressure solution folio (“c”) are
also marked. (Photo is ~1 m wide). D: Photomicrograph (3 mm wide) of matrix
in Trabeg conglomerate at Trabeg (51.32066N, 10.16963W) showing wellpacked quartz (“q”), phyllite (“p”), and other grains, with indented clast
boundaries and pressure solution folia.
Therefore, it is concluded that while the Dingle Group was deformed
and shortened by folding and faulting by the Acadian event, the
pervasive tectonic fabric is largely Variscan and post-lithification in
origin.
REFERENCES CITED
Meere, P.A., and Mulchrone, K.F., 2006, Timing of deformation within Old Red
Sandstone lithologies from the Dingle Peninsula, SW Ireland: Journal of the
Geological Society of London, v. 163, p. 461–469, doi:10.1144/0016-764905099.
Meere, P.A., Mulchrone, K.F., McCarthy, D.J., Timmerman, M.J., and Dewey,
J.F., 2016, Prelithification and synlithification tectonic foliation development
in a clastic sedimentary sequence: Geology, v. 44, p. 291–294,
doi:10.1130/G37587.1.
Todd, S.P., 2015, Structure of the Dingle Peninsula, SW Ireland: Evidence for the
nature and timing of Caledonian, Acadian and Variscan tectonics: Geological
Magazine, v. 152, p. 242–268, doi:10.1017/S0016756814000260.
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