Fluvial Form in Modern Continental Sedimentary Basins: Distributive Fluvial Systems: REPLY

Fluvial form in modern continental sedimentary basins: Distributive fl uvial systems: REPLY REPLY: doi:10.1130/G31588Y.1 rock record and attributed to large fl uvial systems were deposited either as part of a DFS or an axial system fed by DFS. A.J. Hartley1, G.S. Weissmann2, G.J. Nichols3, and L.A. Scuderi2 Sambrook Smith et al. suggest that we do not consider the issue of 1Department of Geology and Petroleum Geology, University of preservation potential, and refer to the half-graben facies model of Gaw- Aberdeen, Aberdeen AB24 3UE, UK thorpe and Leeder (2000). A study of the Palomas half-graben by Mack 2Department of Earth and Planetary Sciences, MSC03 2040, 1 University et al. (2002) noted that the axial ancestral Rio Grande covered a relatively of New Mexico, Albuquerque, New Mexico 87131-0001, USA small part of the basin fi ll, as it was constrained by both hangingwall- and 3Department of Earth Sciences, Royal Holloway, University of London, footwall-derived DFS. Thus, where rock record examples are available, Egham, Surrey TW20 0EX, UK they support our assertion that DFS will dominate the stratigraphic record of fl uvial deposits. We thank Sambrook Smith et al. (2010) for their Comment and the The criteria we present for DFS recognition do have some overlap opportunity to expand on points presented in our paper (Weissmann et al., with those of tributive systems. The radial pattern of channels from an 2010a). For further documentation and discussion of distributive fl uvial apical point is a distinctive characteristic, although we disagree that most systems (DFS) we refer readers to Hartley et al. (2010) and Weissmann workers would consider the DFS we illustrate to be alluvial fans. Impor- et al. (2010b). As noted by Sambrook Smith et al., we avoided discuss- tantly, the downstream decrease in channel size occurs in all climate re- ing marine marginal fl uvial systems. We took this approach because we gimes (not just semi-arid or arid systems) and is an order of magnitude wished to focus on sedimentary basins exempt from direct marine infl u- larger than that described for tributive semi-arid rivers. ence and to remove ambiguity associated with accommodation generated A key point that we maintain is that in order to understand fl uvial by a marine base-level rise. Sambrook Smith et al. suggest that degra- systems preserved in the rock record, it is important when using modern dational terrains are preserved in the rock record and discuss a number analogues to examine areas of the Earth’s crust that are subsiding and of examples. We fully acknowledge the importance of incised valley fi lls will therefore be preserved. We do not wish to imply that “the majority due to sea-level change in the rock record (Weissmann et al., 2010b). It is of current fl uvial facies models are of limited relevance to the interpreta- not clear, however, whether the examples of the ancestral Mississippi and tion of ancient sediments” (Sambrook Smith et al., 2010). At the scale of Tertiary of the Gulf Coast, highlighted by Sambrook Smith et al., were the channels and their fi ll, these descriptions and interpretations provide distributive or tributive in nature. Valley-fi ll deposits are preserved as the a very valuable body of literature. However, what we believe is missing initial phase of aggradation in these basins; however, we would argue that in the literature on fl uvial systems is an understanding of the larger-than- as the basin fi lls, the depositional pattern becomes distributive. We also channel belt and basinal context in which fl uvial systems are developed, argue that, with the exception of incised valley fi lls, fl uvial deposits in and it is at this larger scale that we believe the DFS paradigm has its great- degradational terrains are very rarely preserved within fl uvial basin-fi ll est value. successions, and that DFS dominate the rock record. Sambrook Smith et al. were concerned that large rivers are excluded REFERENCES CITED from our study. Additionally, they make the point that many thick, region- ally extensive fl uvial deposits are preserved in the rock record and sug- Fielding, C.R., 2007, Sedimentology and stratigraphy of large river deposits: Recognition in the ancient record, and distinction from ‘incised valley fi lls,’ gest that the largest system we illustrated, the Pilcomayo, is too small to in Gupta, A., ed., Large Rivers, Geomorphology and Management: Sussex, develop the far larger rivers that they state are found in the rock record. Wiley Interscience, p. 97–113. The authors miss our point here in two ways. First, we did not focus on Gawthorpe, R.L., and Leeder, M.R., 2000, Tectono-sedimentary evolution of specifi c rivers but presented overall depositional patterns observed in con- active extensional basins: Basin Research, v. 12, p. 195–218, doi:10.1046/ tinental sedimentary basins. We observed that many of the large rivers are j.1365-2117.2000.00121.x. Hartley, A.J., Weissmann, G.S., Nichols, G.J., and Warwick, G.L., 2010, Large not present in actively aggrading sedimentary basins and therefore have distributive fl uvial systems: Characteristics, distribution, and controls limited preservation potential. Where large rivers do enter sedimentary on development: Journal of Sedimentary Research, v. 80, p. 167–183, basins, they develop DFS (see examples documented in Hartley et al., doi:10.2110/jsr.2010.016. 2010) and lie in axial positions in the basin (e.g., Brahmaputra and Gan- Mack, G.H., Leeder, M.R., and Salyards, S.L., 2002, Temporal and spatial variability ges Rivers, Himalayan Foreland Basin; Paraná/Paraguay River system, of alluvial-fan and axial-fl uvial sedimentation in the Plio-Pleistocene Palomas half graben, southern Rio Grande rift, New Mexico, U.S.A., in Renaut, G.W., Andes Foreland Basin). Thus, these large rivers may be represented in and Ashley, G.M., eds., Sedimentation in Continental Rifts: The Society for the rock record. Second, the dimensions of fl uvial deposits preserved in Sedimentary Geology Special Publication 73, p. 165–177. the rock record will not equate directly to the present-day dimensions of Sambrook Smith, G.H., Best, J.L., Ashworth, P.J., Fielding, C.R., Goodbred, S.L., and an individual channel system. Thick, laterally extensive fl uvial sediments Prokocki, E.W., 2010, Fluvial form in modern continental sedimentary basins: Distributive fl uvial systems: Comment: Geology, doi:10.1130/G31507C.1. will form as a result of the avulsion/migration of a channel system(s), and Weissmann, G.S., Hartley, A.J., Nichols, G.J., Scuderi, L.A., Olson, M., Buehler, may be one of the reasons why “large” river deposits have not been iden- H., and Banteah, R., 2010a, Fluvial form in modern continental sedimentary tifi ed. For example, it is clear from satellite imagery and fi eldwork that basins: Distributive fl uvial systems: Geology, v. 38, p. 39–42, doi:10.1130/ past shifts of the Pilcomayo fl uvial system have produced an extensive G30242.1. (up to 60-km-wide) belt of fl uvial sands up to 100 km downstream of the Weissmann, G.S., Hartley, A.J., Nichols, G.J., Scuderi, L.A., Olson, M., Buehler, H., and Massengill, L., 2010b, Alluvial facies distributions in continental apex. Further, avulsion and amalgamation with adjacent DFS will produce sedimentary basins—Distributive fl uvial systems, in Davidson, S., North, sheet-like sandstone bodies with dimensions greater than those discussed C., and Leleu, S., eds., Rivers to Rock Record: The Society for Sedimentary by Fielding (2007). We argue that most sandstone bodies described in the Geology Special Publication (in press). © 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY FORUM, December 2010 e231 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/38/12/e231/3538968/e231.pdf by guest on 24 September 2021.

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