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Holocene Alluvial-Fan Development in the Macgillycuddy's Reeks, Southwest Ireland

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Holocene Alluvial-Fan Development in the Macgillycuddy's Reeks, Southwest Ireland Holocene alluvial-fan development in the Macgillycuddy's Reeks, southwest Ireland E. Anderson School of Geography and Environmental Management, Middlesex University, Queensway, En®eld EN3 4SF, UK S. Harrison* Centre for Quaternary Science, Geography, NES, Coventry University, Priory Street, Coventry CV1 5FB, UK D.G. Passmore Department of Geography, University of Newcastle-upon-Tyne, Newcastle-upon-Tyne NE1 8ST, UK T.M. Mighall Centre for Quaternary Science, Geography, NES, Coventry University, Priory Street, Coventry CV1 5FB, UK ABSTRACT probably a function of both anthropogenic British Isles, the majority of alluvial fans and and climatic forcing. A sequence of events debris cones are deeply incised, partially or Holocene alluvial landforms in the Mac- may have involved initial slope destabili- wholly vegetated, and show little sign of re- gillycuddy's Reeks, southwest Ireland, were zation due to overgrazing and removal of cent activity (Brazier and Ballantyne, 1989). investigated to determine the controls and vegetation that was followed by debris mo- Research on the chronology and paleoenviron- timing of postglacial geomorphic activity. bilization and fan aggradation during in- mental signi®cance of these landforms has Detailed geomorphologic analysis of three tense rainstorms associated with climate demonstrated widespread evidence of Holo- alluvial-fan and debris cones within high- change. cene surface aggradation and incision. This level cirque basins demonstrates evidence activity was particularly intense during the Keywords: alluvial fans, Holocene, south- of episodic phases of late Holocene surface tenth to twelfth centuries (Harvey et al., 1981; west Ireland. aggradation and incision. Radiocarbon Harvey and Renwick, 1987; Tipping and Hal- dates from peat horizons above and below INTRODUCTION liday, 1994) and sixteenth to nineteenth cen- inorganic units show that phases of aggra- turies (Innes, 1983; Brazier et al., 1988; dation cluster into two distinct periods, the Alluvial fans and debris cones are fan- Brazier and Ballantyne, 1989). ®rst after 230±790 calibrated (cal.) yr A.D. shaped bodies of sediment which form at the Despite the body of evidence cited previ- and the second from 1510 cal. yr A.D. to foot of mountain escarpments, slope gullies, ously, uncertainties remain regarding several the present. An additional phase of fan ag- and tributary valleys by rapid deposition from key research issues. First, discerning the re- gradation at one site is dated after 1040± channelized or uncon®ned water¯oods, tran- spective roles of climate and land-use changes 1280 cal. yr A.D. All three phases coincide sitional ¯ows, and debris ¯ows (Blair and Mc- (external forces) in controlling geomorphic ac- with episodes of enhanced late Holocene Pherson, 1994). They can be mutually distin- tivity remains a contentious issue (Ballantyne, valley-¯oor alluviation and debris-¯ow ac- guished on the basis of surface gradient and 1991a, 1991b). Alluvial records in upland ba- tivity from upland Britain. Alluvial fans bulk composition. Alluvial fans commonly sins are commonly incomplete, and materials consist of a variety of sediment facies types and debris cones have developed primarily suitable for dating are commonly lacking. and display surface gradients generally be- as a result of the resedimentation of late Consequently, these proxy records are seldom tween 28 and 128 (Blair and McPherson, Midlandian (Wisconsin) drift and talus of suf®cient resolution to isolate a de®nitive 1994). In contrast, debris cones are generally slopes, and mobilization of materials in- climatic or cultural signal. Second, studies steeper, with average gradients typically in the volved ¯ooding, transitional-¯ow, and de- range of 128±258 (Brazier et al., 1988; Blair have focused on single sites where geomor- bris-¯ow processes. Pollen analysis of peat and McPherson, 1994), and include predomi- phic responses to ¯uxes in external boundary horizons interbedded with alluvial-fan and nantly superposed debris-¯ow lobes (Data Re- conditions may be dif®cult to differentiate debris-cone sediments indicates that land- pository Table DR11). In upland areas of the from internal stochastic or complex response use changes were an important factor in (Schumm, 1973). lowering the threshold for local slope ero- 1GSA Data Repository item 2000112, description In the Macgillycuddy's Reeks, alluvial fans sion. Phases of aggradation also coincide and schematic pro®les of alluvial-fan depositional and debris cones are a common feature at the with well-documented episodes of climate facies, is available on the Web at http:// base of incised-valley and cirque slopes and change, and, hence, fan development is www.geosociety.org/pubs/ft2000.htm. Requests re¯ect widespread slope destabilization during may also be sent to Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301; e-mail: the late Quaternary drift and talus covers. *E-mail: [email protected]. [email protected]. Three of these alluvial-fan and debris-cone GSA Bulletin; December 2000; v. 112; no. 12; p. 1834±1849; 11 ®gures; 2 tables; Data Repository item 2000112. 1834 HOLOCENE ALLUVIAL FAN DEVELOPMENT, SOUTHWEST IRELAND complexesÐlocated in the Coomloughra distinct feeder channels incised into Younger downstream washout debris from transitional Glen, Hag's Glen, and Curraghmore cirque ba- Dryas till and is overlain by talus (Fig. 2). The ¯ows that deposited a series of stacked lobes sins (Fig. 1 and Table 1)Ðcontain buried peat proximal reach consists of superposed vis- and lobes on fan A. Fan aggradation was fol- horizons and hence facilitate an opportunity to cous-debris and transitional-¯ow lobes and lowed by renewed peat development, although address some of the research issues previously splays, which grade proximally into poorly surface stability was interrupted brie¯y by the outlined. Accordingly, the principal aims of sorted channel ®lls. Distally, dilute debris deposition of channelized sandy gravel in the this paper are to (1) establish the geomorphic ¯ows and ¯uvial boulder lobes replace these distal reach (LFA CG4, section 2). The initial development and chronology of alluvial fans facies, and the distal portion is made up of an timing of this episode of peat formation is un- and debris cones and (2) determine the envi- expansive assemblage of superposed ¯uvial certain, but the thickness of the peat layer in ronmental controls responsible for periods of cobble bars and sheet¯ood deposits. Five section 1 (0.28 m; Fig. 3) indicates a pro- landform stability and accelerated geomorphic stream-cut exposures (sections 1±5; Fig. 3) re- longed period of surface stability prior to the activity. The methodology employed in this veal that this surface assemblage, referred to return of alluvial deposition after 1510±1930 study is described in the Appendix. here as lithofacies association (LFA) CG5, cal. yr A.D. Renewed slope instability after consists of clast-imbricated, gravel-cobble and 1510±1930 cal. yr A.D. caused the deposition STUDY AREA cobble-boulder deposits, which ®ne distally of fan B and possibly transitional-¯ow depo- into a sequence of laminated silts and sands sition on fan A. At this time, the principal The Macgillycuddy's Reeks is a high-relief and are overlain by sand-and-cobble gravel. basin stream ¯owed around the outer margin massif, consisting of Devonian sandstones and Interstrati®ed peats and clastic units underlie of fan B (Fig. 2). Following the cessation of shales (Fig. 1). The legacy of Pleistocene gla- LFA CG5 in sections 1 and 2 (Fig. 3). Radio- fan aggradation, however, erosion by fan B ciations has left the massif deeply dissected carbon dating of peat samples from LFAs surface streams and localized cobble-bar de- by several glacial troughs and numerous, well- CG2 and CG4 produced dates of 1510±1930 position appear to have caused the avulsion of developed cirque basins. The last phase of val- cal. yr A.D. and 650±790 cal. yr A.D., re- the principal basin stream, leading to the for- ley glaciation occurred during the late Mid- spectively (Table 2). Uprooted vegetation in mation of the present-day channel. landian Glenavy Stadial (26±13 ka). The feeder-channel deposits demonstrates that the Younger Dryas cooling (11±10 ka) also pro- fan is still partially active. The Hag's Glen Alluvial Fan moted the development of six small cirque Geomorphic relationships between fan A glaciers (Anderson et al., 1998). The present and fan B are inconclusive (Fig. 2), but strati- Geomorphology and Stratigraphy climate of this area is temperate maritime, and graphic evidence appears to indicate that at The Hag's Glen alluvial fan is located just the region is frequently affected by the pas- least part of fan A predates fan B. Paleocur- south of Lough Callee (Fig. 4) at the foot of sage of midlatitude low-pressure systems. rent analysis of the buried alluvial units (LFAs a gully cut into Glenavy Stadial till. The prox- Consequently, the weather is often stormy, CG3 and CGi) exposed in sections 1±5 dem- imal reach of the fan consists of several tran- and heavy rain is common throughout the onstrates that they were deposited by water- sitional-¯ow boulder lobes. Although the mid- year, particularly in the mountains, where an- ¯oods derived from the catchment area of fan fan area is heavily vegetated, close inspection nual precipitation probably exceeds 2500 mm. A and probably, therefore, represent the down- reveals an expansive assemblage of super- stream washout debris of transitional ¯ows posed stream¯ow boulder bars. Wells and Coomloughra Glen Alluvial-Fan and which formed fan A (Figs. 2 and 3). Presently, Harvey (1987) pointed out that ¯uvial boulder Debris-Cone Complex it is not clear at what stage during the devel- bars could be interpreted as sieve deposits (cf. opment of fan A that these alluvial units were Hooke, 1967). The boulder bars in the Hag's Geomorphology and Stratigraphy deposited because this fan probably consists Glen alluvial fan, however, overlie peat de- The alluvial-fan complex within Coom- of two temporally distinct assemblages of su- posits that would prevent the rapid surface- loughra Glen consists of two distinct fans, re- perposed transitional-¯ow lobes.
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