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doi:10.1130/ G37387Y.1
Supercritical-flow structures on a Late Carboniferous delta front: sedimentologic and
paleoclimatic significance
Dario Ventra1, Matthieu J.B. Cartigny2, Jochem F. Bijkerk3,
4
and Sanem Açikalin
1
Faculty of Geosciences, Utrecht University, 3584CS, Utrecht, the
Netherlands
National Oceanography Centre, European Way, Southampton,
SO14 3ZH, UK
3
British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK
4
Badley Ashton & Associates Ltd., Winceby House, Horncastle, LN9 6PB,
UK
2
We appreciate this discussion as confirmation of the novelty that
cyclic steps (and their recognition) represent for clastic sedimentology,
eliciting further research. Kane and Hodgson (2015) interpret centimeterscale structures, erosional surfaces, and local sediment deformation as
evidence of successive depositional events that progressively infilled
fluvial scours associated with a channel confluence. While a hypothesis
of deposition by separate events is still in consideration on our part,
evidence clearly suggests cyclic-step aggradation as the depositional
mechanism for the peculiar stratal geometry at Derby Delph. The
rhythmic, conformable architecture does not match those described for
migrating fluvial confluences, where scouring and the juxtaposition of
different facies are common (e.g., Bristow et al., 1993; Best and Rhoads,
2008). The lack of vertical facies transitions fits neither the changing
sediment-flow interactions within a deep channel (~15–20 m) nor the
internal structure of side bars, which would be assembled by far thinner
bedsets. In our article (Ventra et al., 2015), we do interpret cross-bedded
deposits immediately superposed to Derby Delph strata as the fluvial
“topset,” in agreement with Hampson’s (1997) analysis of the Lower
Kinderscout Grit. Hampson (1997, p. 281, 2nd column) understandably
noted that “cosets of giant cross-beds and undulatory bedding” would not
fit a typical deltaic interpretation, adhering to previously proposed fluvial
models. Recent insights on supercritical-flow bedforms support our
depositional model. By way of example, we point to the identity in scale
and architecture between Derby Delph strata (Figs. 1A and 1B) and those
of a glacially fed delta front in Yukon (Gilbert and Crookshanks, 2009;
Fig. 1C), where hyperpycnal flows form giant upslope-migrating
bedforms with stoss-side deposition, matching our process interpretation.
Regarding small-scale sedimentary structures, reported in our article,
we remark again that they are so scarce as to become irrelevant to an
interpretation of large-scale depositional mechanics. As mentioned in our
paper, fluctuations in flow regime and the migration of ephemeral flow
cells associated with hydraulic jumps may superpose ripples and
antidunes to the aggrading interface. The whole stratal architecture is
conformable, with significant erosion limited to high-angle surfaces we
interpret as set boundaries. Ichnofossils are reported from shallowmarine sediments of the Pennine Basin (e.g., Brettle et al., 2002), so their
absence might be significant to assess sustained aggradation. Centimetric
scours and deformation by loading are indeed expected (and locally
occur) in strata related to rapid sediment dumping by a pulsating flow on
a subaqueous front, but they are neither extensive nor randomly
distributed, suggesting depositional continuity and process unity that fit
our interpretation.
Figure 1. Panoramic view (A) and interpretation (B) of part of the Derby
Delph outcrop, offering a typical example of the basic stratal architecture;
paleoflow to the left. C: Detail from Gilbert and Crookshanks (2009, their
figure 9b), showing a subbottom acoustic transect from the modern delta
front of the Slims River (Yukon), reoriented to match paleoflow direction in A
and B above; black inset highlights the identity in scale and geometry with
stratal relationships at Derby Delph (B).
REFERENCES CITED
Best, J.L., and Rhoads, B.L., 2008. Sediment transport, bed morphology and the
sedimentology of river channel confluences, in Rice, S.P. et al., eds., River
Confluences, Tributaries and the Fluvial Network: New York, John Wiley &
Sons Ltd., p. 45–72, doi:10.1002/9780470760383.ch4.
Brettle, M.J., McIlroy, D., Elliott, T., Davies, S.J., and Waters, C.N., 2002.
Identifying cryptic tidal influences within deltaic successions: An example
from the Marsdenian (Namurian) interval of the Pennine Basin, UK: Journal
of the Geological Society, London, v. 159, p. 379–391, doi:10:1144/0016–
764901–070.
Bristow, C.S., Best, J.L., and Roy, A.G., 1993. Morphology and facies models of
channel confluences, in Marzo, M., and Puigdefábregas, C., eds., Alluvial
Sedimentation: International Association of Sedimentologists Special Publication 17, p. 91–100, doi:101002/9781444303995.ch8.
Gilbert, R., and Crookshanks, S., 2009, Sediment waves in a modern high-energy
glacilacustrine environment: Sedimentology, v. 56, p. 645–659, doi:10.1111
/j.1365-3091.2008.00990.x.
Hampson, G.J., 1997, A sequence stratigraphic model for deposition of the Lower
Kinderscout Delta, an Upper Carboniferous turbidite-fronted delta: Proceedings of the Yorkshire Geological Society, v. 51, p. 273–296, doi:10.1144
/pygs.51.4.273.
Kane, I., and Hodgson, D.M., 2015, Supercritical-flow structures on a Late
Carboniferous delta front: Sedimentologic and paleoclimatic significance:
Comment: Geology, v. 43, p. e374.
Ventra, D., Cartigny, M.J.B., Bijkerk, J.F., and Acikalin, S., 2015, Supercriticalflow structures on a Late Carboniferous delta front: Sedimentologic and
paleoclimatic significance: Geology, v. 43, p. 731–734, doi:10.1130
/G36708.1.
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Geology
Supercritical-flow structures on a Late Carboniferous delta front: sedimentologic
and paleoclimatic significance: REPLY
Dario Ventra, Matthieu J.B. Cartigny, Jochem F. Bijkerk and Sanem Açikalin
Geology 2015;43;e375
doi: 10.1130/G37387Y.1
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