Megaflutes and Deepwater Channels
Photos by Christopher Kendall and Peter Haughton.
On the Atlantic shore of Ross Bay just north of the Shannon Estuary in Co. Clare, the Ross Formation
outcrops. These rocks expose deepwater fan lobes that exhibit a combination of megaflutes and eroded channels whose fill spills out onto the overbank areas in the form of wings (Elliot, et al, 2000). Bedding plane
s are often rippled and the same surfaces
have megaflutes eroded into them. The character of the fill of megaflutes suggest deposition of the overbank area was of finer thin bedded shale
prone sediments than that of the sand fill of the channels.
A variety of depositional models have been proposed to explain the larger scale architecture of the Ross system and that of Ross Bay. Most workers agree that the system has both progradational to aggradational trends, punctuated by episodes of fan shutdown, with more sheet-like geometries at the base replaced upwards by a combination of shallow channels and sheets reflecting greater erosion and a more proximal position overall. This is consistent with the longer term basin-fill evolution from deep-water to slope to shallow water deltaic deposition. Early flows in the basal Ross Formation were characterized by hybrid rheology, but the bulk of the formation is characterized by conventional turbidites, albeit dominated by ungraded, largely structureless sandstone beds with ripple-laminated tops reflecting the limited grain size range of the input sediment. Many of the sheet-like beds higher in the formation are probably lateral wings to channels, implying the flows were able to overspill relatively easily. Shallow (a few meters to c. 10 m deep, up to a few 100 m wide) probably distributative channels lacking levees fed sediment to frontal and lateral splays.
Elliott (2000a,b) suggested the megaflute surfaces were key to understanding facies distribution. These surfaces were related to single large bypassing flows that carved megaflutes and through differential erosion created channels (Fig. 1). These outsized flows presumably also deposited thick sands deeper in the basin. The channels created subsequently focused flows, promoting further erosion and preventing all but fine-grained overspill onto the megaflute surface (producing the so-called fine-grained ‘repair’ facies). As the channels filled, increasingly sandy parts of the flows were able to spread more widely, creating channel wings that thicken and become more sand-prone upwards.
Fig. 1. Elliott's 2000 depositional relationships of the Ross Formation megaflutes within the interpreted fan systems outcropping around the axis of the Shannon Estuary.
Lien et al. (2003) downplayed the significance of the megaflutes in channel initiation suggesting that they do not always occur on single surfaces. They also recognized common thickening-upward cycles in parts of the upper Ross and interpreted these as a combination of lateral channel migration (not a feature of the Elliott model) and again increased spillover as channels aggraded and filled (Fig. 2). The megaflutes were related to scouring produced at the channel margins, particularly where flows were overspilt to lateral splays or ‘spillover lobes’ as the channels filled. They also used detailed palaeocurrent measurements to infer that groups of channels and their spillovers occurred in sinuous belts within the poorly packaged turbidites at the largest length scales.
Fig. 2. Lien et al. (2003) depositional relationships of the Ross Formation megaflutes within the interpreted fan systems outcropping around the axis of the Shannon Estuary.
A third model (Fig. 3) appeals to likely growth and retreat of distributative channel networks on the fan surface and recognizes that some of the sheets are frontal rather than lateral splays connected up-dip with channels. In this case, the megafluted surfaces would represent scouring in the channel-lobe transition zone (cf. Chapin et al. 1994). As they commonly occur beneath fine grained ‘repair’ facies, this suggests they are cut as the feeding channel is abandoned, perhaps in response to lobe aggradation to the point where it forces up-dip deposition and backfilling, allowing under capacity flows to accelerate across and scour the downdip splay. If correct, this would imply most of the sand is caught up-dip in the channels, and has not bypassed down-dip.
Fig. 3. Chapin et al. (1994) depositional relationships of the Ross Formation megaflutes within the interpreted fan systems outcropping around the axis of the Shannon Estuary.
Chapin, M.A., Davies, P., Gibson, J.L. & Pettingill, H.S. (1994), Reservoir architecture of turbidite sheet sandstones in laterally extensive outcrops, Ross Formation, western Ireland. In Weimer, P., Bouma, A.H. & Perkins, R.F (eds), Submarine fans and turbidite systems, GCSSEPM Foundation 15th Annual Research Conference, 53-68.
Elliott, T., 2000 a, Depositional architecture of a sand-rich, channelized turbidite system: the Upper Carboniferous Ross Sandstone Formation, Western Ireland. in P. Weimer, R. M. Slatt, A. H. Bouma, and D. T. Lawrence, eds., Deep-water reservoirs of the world: Gulf Coast Section SEPM Foundation, Twentieth Annual Research Conference, p. 342–373.
Elliott, T., 2000 b, Megaflute erosion surfaces and the initiation of turbidite channels,
Geology; v. 28; no. 2; p. 119–122.
Lien T., Walker, R. G., and Martinsen, O. J. (2003) Turbidites in the Upper Carboniferous Ross Formation, Western Ireland -
reconstruction of a sinuous channel and sandy spillover system, Sedimentology,
Martinsen, O.J. (1989), Styles of soft sediment deformation on a Namurian (Carboniferous) delta slope, western Ireland Namurian Basin, Ireland. In Whatley, M.K.G. & Pickering, K.T. (eds) Deltas: sites and traps for fossil fuels, Geological Society Special Publication, 41, 167-177.