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Gravel imbrication on the deflating backshores of beaches on Prince Edward Island, Canada
Sedimentary Geology, ELSEVIER
Sedimentary Geology 101 (1996) 145-148
Short Note
Gravel imbrication on the deflating backshores of beaches on Prince Edward Island, Canada Lawrence H. Tanner Department of Geography and Earth Science, Bloomsburg University, Bloomsburg, PA 17815, USA
Received 2 December 1994; revised version accepted 15 February 1995
Abstract Discoidal- to prolate-shaped pebble- to cobble-size clasts on the partially deflated beach backshore of the north shore of Prince Edward Island display a pronounced imbrication. The direction of imbrication correlates with the orientation of wind-shadow sand ridges behind the clasts. The imbrication is created by the selective removal of sand particles beneath the upwind margins of the clasts during deflation. This fabric can be preserved and has the potential to provide a means for the measure of paleowind orientation in partially deflated deposits in shoreline and possibly interdune areas. I. Introduction Clast imbrication, the upcurrent dip of flattened clasts, is a widely used paleocurrent indicator typically associated with clast-supported conglomerates deposited by streamflow, but also may include conglomerates deposited by wave action, turbidity and density currents, and debris-flow processes (Bluck, 1967; Collinson and Thompson, 1982; N e m e c and Steel, 1984). Imbrication of clasts is not reported in the literature from modern eolian deposits (Cooke and Warren, 1973; Mabbutt, 1977) or on ancient interdune surfaces (Ahlbrandt and Fryberger, 1981). This p a p e r documents the occurrence of clast imbrication on a partially deflated beach backshore.
2. Setting Extensive beaches along the north shore of Prince Edward Island, Canada, formed by re-
working of till and erosion of the relatively soft sandstones, siltstones and mudstones of the Prince Edward Island Redbed Series of CarboniferousPermian age (Poll, 1983). These beaches are currently in retreat (McCann and Bryant, 1972). Sediment from erosion of rocky headlands and backshore beach ridges is distributed by wave energy from the Gulf of St. Lawrence to beaches and spits along the shoreline. Pebble- to boulder-size clasts are transported during storms alongshore and upshore and periodically deposited on the backshore of the beaches. The predominant wind direction for Prince Edward Island varies in an arc from south to northwest (Bryant and McCann, 1972). Beach ridges along the north shore shelter the beach from southerly winds, resulting in a local mean wind direction along the beach of west to southwest, subparallel to the shoreline. These winds can be quite strong. During a period of mild summer weather, winds were observed gusting at
0037-0738/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0037-0738(95)00019-4
146
L.H. Tanner~Sedimentary Geology 101 (1996) 145-148
3. Clast fields
Fig. 1. Clast field on the backshore near Covehead Bay inlet displaying clear imbrication. Scale in middle ground (arrow) is approximately 16 cm. Eroded vegetated beach ridge in background is covered by a wedge of modern eolian sediment.
Clast fields 5 to 20 m wide are common in the backshore on beaches in Prince Edward Island National Park. These clast fields extend from the storm berm to the base of the beach ridges (Fig. 1). The location of the clast fields does not appear to be controlled by proximity to the rocky headlands. Clasts ranging in size from pebble to cobble cover 2 0 - 4 0 % of the surface. The intervening surface is covered by fine sand. Low-sinuous bars, averaging 2 m long x 0.5 m wide x 0.1 m high, occur near the edge of the storm berm, oriented parallel to the berm. Clasts on the berm edge and on these bars are typically aligned parallel to the sediment surface.
4. Ciast imbrication velocities exceeding 30 k m / h . Sand was transported as a sheet, centimeters above the beach during gusts. The sand was deposited in low-amplitude ripples between gusts. Eolian deposition also takes place on the eroded shoreward face of older vegetated beach ridges as shoreward-sloping wedges (Fig. 1). Sand is transported as low-amplitude dip-parallel ripples on the flanks of the wedges.
A substantial portion of siltstone and sandstone clasts with discoidal, bladed or prolate shape (as defined by Zingg, 1935) on the flat backshore (behind the storm berm) dip at angles from 8 ° to 35 °, averaging 22 ° (Fig. 2). Imbrication of varying degrees of development was observed at six locations along a 15 km section of beach within Prince Edward Island National Park. Imbricated clast fields range from approximately 10 m to 50 m long (parallel to shore). Imbrication was observed in clasts ranging in size from 3 × 4 cm (intermediate:long axes) to 12 × 15 cm with the intermediate axis most commonly parallel to
X~ N L S
Fig. 2. Detail of clast field shown in Fig. 1. Scale bar (arrow) divisions are 1 cm.
= = =
263° 50 86% 33 °
X~ N L S
= 273 ° = 50 : 98% = 13 °
Fig. 3. Equal-area current rose plots for azimuth of dip of imbricated clasts and azimuth of sand ridges on the downwind margin of clasts measured near Covehead Bay inlet. L = consistency ratio. Both vector m e a n s are statistically significant by the Rayleigh test.
L.H. Tanner / Sedimentary Geology 101 (1996) 145-148
Fig. 4. Imbricated clasts with sand ridges that extend from the downwind margin.
the dip direction, although clasts are also oriented with the long axis parallel to dip. The orientation of the a:b plane was measured for 50 clasts in a well imbricated clast field near the inlet to Covehead Bay. "The mean azimuth of imbrication for this group is 262.7 ° with a standard deviation of 32.8 ° (Fig. 3). Many clasts were also observed to have a sharp ridge of sand with a relief of 1 to 2 cm extending up to 11 cm from the up-tilted margin of a clast (Fig. 4). M e a s u r e m e n t at the same location of the orientation of the
Fig. 5. Tilted clast (center) displaying arcuate scour pit bordering upwind margin.
147
sand ridges of 50 clasts, selected to be free of interference from adjacent clasts, yields a mean azimuth of 272.5 ° with a standard deviation of 12.5 ° (Fig. 3). An arcuate-shaped erosional scour pit up to 3 cm deep occurs at the upwind face of many clasts (Fig. 5). The clast imbrication was developed by scouring of sand beneath the upwind margins of clasts, lowering the margins. This mechanism has been shown experimentally to cause tilting and toppling of clasts during ventifact formation (Sharp, 1964, 1980) and may be partially responsible for the formation of mature deflation surfaces, or desert pavements (Breed et al., 1989). The close correlation of the azimuth of imbrication and the wind-shadow ridges downwind of the clasts confirms this interpretation.
5. Discussion These clast fields contrast with imbricate clast-supported beach gravels seaward of a berm (Bluck, 1967). The clast fields on Prince Edward Island are storm lags subsequently modified by eolian deflation. These clast fields are not mature, or "wind stable" deflation surfaces. An abundant sediment supply from the backshore and beach now prevents lowering of the beach surface. An increase in sediment input (progradation of the beach) would preserve this fabric. Erosional scarps on the beach ridges in a few locations expose in cross-sections clast layers with crude imbrication. A variety of stratification types can form in backshore and interdune settings from eolian as well as noneolian processes making paleowind interpretation difficult. Movement of sand may occur in windblown sheets, wind-driven ripples, migrating dunes, storm washovers, ephemeral stream channels, or sheet flow. Interpretation of transport process and direction may be difficult where these processes have operated in combination. The clast fabric described here has potential for use as an additional means for recognizing deflationary surfaces and as a paleocurrent indicator.
148
L.H. Tanner / Sedimentary Geology 101 (1996) 145-148
Acknowledgements Field expenses for this project were supported by Bloomsburg University. The manuscript benefitted from editorial suggestions by J.F. Hubert.
References Ahlbrandt, T.S. and Fryberger, S.G., 1981. Sedimentary features and significance of interdune deposits. In: F.G. Ethridge and R.M. Flores (Editors), Recent and Ancient Nonmarine Depositional Environments: Models for Exploration. Soc. Econ. Paleontol. Mineral. Spec. Publ., 31: 293-314. Bluck, B.J., 1967. Sedimentation of beach gravels: examples from South Wales. J. Sedement. Petrol., 27: 128-156. Breed, C.S., McCauley, J.F. and Whitney, M.I., 1989. Wind erosion forms. In: D.S.G. Thomas (Editor), Arid Zone Geomorphology. Halsted Press, New York, N.Y., pp. 284307. Bryant, E.A. and McCann, S.B., 1972. A note on wind and
wave conditions in the southern part of the Gulf of St. Lawrence. Marit. Sediment., 8: 101-103. Collinson, J.D. and Thompson, D.B., 1982. Sedimentary Structures. Allen and Unwin, London, 194 pp. Cooke, R.U. and Warren, A., 1973. Geomorphology in Deserts. Univ. California Press, Berkeley, Calif., 374 pp. Mabbutt, J.A., 1977. Desert Landforms. MIT Press, Cambridge, Mass., 340 pp. McCann, S.B. and Bryant, E.A., 1972. Barrier islands, sand spits and dunes in the southern Gulf of St. Lawrence. Marit. Sediment., 8: 104-106. Nemec, W. and Steel, R.J., 1984. Alluvial and coastal conglomerates: their significant features and some comments on gravelly mass-flow deposits. In: E.H. Koster and R.J. Steel (Editors), Sedimentology of Gravels and Conglomerates. Can. Soc. Pet. Geol., Mem., 10: 1-31. Poll, H.W. van de, 1983. Geology of Prince Edward Island. Department of Energy and Forestry Report 83-1, Charlottetown, Prince Edward Island, 66 pp. Sharp, R.P., 1964. Wind-driven sand in Coachella Valley, California. Geol. Soc. Am. Bull., 75: 785-804. Sharp, R.P., 1980. Wind-driven sand in Coachella Valley, California: further data. Geol. Soc. Am. Bull., 91: 724-730. Zingg, T., 1935. Beitr~ige zur Schotteranalyse. Schweiz. Mineral. Petrogr. Mitt., 15: 39-140.
Sedimentary Geology 101 (1996) 145-148
Short Note
Gravel imbrication on the deflating backshores of beaches on Prince Edward Island, Canada Lawrence H. Tanner Department of Geography and Earth Science, Bloomsburg University, Bloomsburg, PA 17815, USA
Received 2 December 1994; revised version accepted 15 February 1995
Abstract Discoidal- to prolate-shaped pebble- to cobble-size clasts on the partially deflated beach backshore of the north shore of Prince Edward Island display a pronounced imbrication. The direction of imbrication correlates with the orientation of wind-shadow sand ridges behind the clasts. The imbrication is created by the selective removal of sand particles beneath the upwind margins of the clasts during deflation. This fabric can be preserved and has the potential to provide a means for the measure of paleowind orientation in partially deflated deposits in shoreline and possibly interdune areas. I. Introduction Clast imbrication, the upcurrent dip of flattened clasts, is a widely used paleocurrent indicator typically associated with clast-supported conglomerates deposited by streamflow, but also may include conglomerates deposited by wave action, turbidity and density currents, and debris-flow processes (Bluck, 1967; Collinson and Thompson, 1982; N e m e c and Steel, 1984). Imbrication of clasts is not reported in the literature from modern eolian deposits (Cooke and Warren, 1973; Mabbutt, 1977) or on ancient interdune surfaces (Ahlbrandt and Fryberger, 1981). This p a p e r documents the occurrence of clast imbrication on a partially deflated beach backshore.
2. Setting Extensive beaches along the north shore of Prince Edward Island, Canada, formed by re-
working of till and erosion of the relatively soft sandstones, siltstones and mudstones of the Prince Edward Island Redbed Series of CarboniferousPermian age (Poll, 1983). These beaches are currently in retreat (McCann and Bryant, 1972). Sediment from erosion of rocky headlands and backshore beach ridges is distributed by wave energy from the Gulf of St. Lawrence to beaches and spits along the shoreline. Pebble- to boulder-size clasts are transported during storms alongshore and upshore and periodically deposited on the backshore of the beaches. The predominant wind direction for Prince Edward Island varies in an arc from south to northwest (Bryant and McCann, 1972). Beach ridges along the north shore shelter the beach from southerly winds, resulting in a local mean wind direction along the beach of west to southwest, subparallel to the shoreline. These winds can be quite strong. During a period of mild summer weather, winds were observed gusting at
0037-0738/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0037-0738(95)00019-4
146
L.H. Tanner~Sedimentary Geology 101 (1996) 145-148
3. Clast fields
Fig. 1. Clast field on the backshore near Covehead Bay inlet displaying clear imbrication. Scale in middle ground (arrow) is approximately 16 cm. Eroded vegetated beach ridge in background is covered by a wedge of modern eolian sediment.
Clast fields 5 to 20 m wide are common in the backshore on beaches in Prince Edward Island National Park. These clast fields extend from the storm berm to the base of the beach ridges (Fig. 1). The location of the clast fields does not appear to be controlled by proximity to the rocky headlands. Clasts ranging in size from pebble to cobble cover 2 0 - 4 0 % of the surface. The intervening surface is covered by fine sand. Low-sinuous bars, averaging 2 m long x 0.5 m wide x 0.1 m high, occur near the edge of the storm berm, oriented parallel to the berm. Clasts on the berm edge and on these bars are typically aligned parallel to the sediment surface.
4. Ciast imbrication velocities exceeding 30 k m / h . Sand was transported as a sheet, centimeters above the beach during gusts. The sand was deposited in low-amplitude ripples between gusts. Eolian deposition also takes place on the eroded shoreward face of older vegetated beach ridges as shoreward-sloping wedges (Fig. 1). Sand is transported as low-amplitude dip-parallel ripples on the flanks of the wedges.
A substantial portion of siltstone and sandstone clasts with discoidal, bladed or prolate shape (as defined by Zingg, 1935) on the flat backshore (behind the storm berm) dip at angles from 8 ° to 35 °, averaging 22 ° (Fig. 2). Imbrication of varying degrees of development was observed at six locations along a 15 km section of beach within Prince Edward Island National Park. Imbricated clast fields range from approximately 10 m to 50 m long (parallel to shore). Imbrication was observed in clasts ranging in size from 3 × 4 cm (intermediate:long axes) to 12 × 15 cm with the intermediate axis most commonly parallel to
X~ N L S
Fig. 2. Detail of clast field shown in Fig. 1. Scale bar (arrow) divisions are 1 cm.
= = =
263° 50 86% 33 °
X~ N L S
= 273 ° = 50 : 98% = 13 °
Fig. 3. Equal-area current rose plots for azimuth of dip of imbricated clasts and azimuth of sand ridges on the downwind margin of clasts measured near Covehead Bay inlet. L = consistency ratio. Both vector m e a n s are statistically significant by the Rayleigh test.
L.H. Tanner / Sedimentary Geology 101 (1996) 145-148
Fig. 4. Imbricated clasts with sand ridges that extend from the downwind margin.
the dip direction, although clasts are also oriented with the long axis parallel to dip. The orientation of the a:b plane was measured for 50 clasts in a well imbricated clast field near the inlet to Covehead Bay. "The mean azimuth of imbrication for this group is 262.7 ° with a standard deviation of 32.8 ° (Fig. 3). Many clasts were also observed to have a sharp ridge of sand with a relief of 1 to 2 cm extending up to 11 cm from the up-tilted margin of a clast (Fig. 4). M e a s u r e m e n t at the same location of the orientation of the
Fig. 5. Tilted clast (center) displaying arcuate scour pit bordering upwind margin.
147
sand ridges of 50 clasts, selected to be free of interference from adjacent clasts, yields a mean azimuth of 272.5 ° with a standard deviation of 12.5 ° (Fig. 3). An arcuate-shaped erosional scour pit up to 3 cm deep occurs at the upwind face of many clasts (Fig. 5). The clast imbrication was developed by scouring of sand beneath the upwind margins of clasts, lowering the margins. This mechanism has been shown experimentally to cause tilting and toppling of clasts during ventifact formation (Sharp, 1964, 1980) and may be partially responsible for the formation of mature deflation surfaces, or desert pavements (Breed et al., 1989). The close correlation of the azimuth of imbrication and the wind-shadow ridges downwind of the clasts confirms this interpretation.
5. Discussion These clast fields contrast with imbricate clast-supported beach gravels seaward of a berm (Bluck, 1967). The clast fields on Prince Edward Island are storm lags subsequently modified by eolian deflation. These clast fields are not mature, or "wind stable" deflation surfaces. An abundant sediment supply from the backshore and beach now prevents lowering of the beach surface. An increase in sediment input (progradation of the beach) would preserve this fabric. Erosional scarps on the beach ridges in a few locations expose in cross-sections clast layers with crude imbrication. A variety of stratification types can form in backshore and interdune settings from eolian as well as noneolian processes making paleowind interpretation difficult. Movement of sand may occur in windblown sheets, wind-driven ripples, migrating dunes, storm washovers, ephemeral stream channels, or sheet flow. Interpretation of transport process and direction may be difficult where these processes have operated in combination. The clast fabric described here has potential for use as an additional means for recognizing deflationary surfaces and as a paleocurrent indicator.
148
L.H. Tanner / Sedimentary Geology 101 (1996) 145-148
Acknowledgements Field expenses for this project were supported by Bloomsburg University. The manuscript benefitted from editorial suggestions by J.F. Hubert.
References Ahlbrandt, T.S. and Fryberger, S.G., 1981. Sedimentary features and significance of interdune deposits. In: F.G. Ethridge and R.M. Flores (Editors), Recent and Ancient Nonmarine Depositional Environments: Models for Exploration. Soc. Econ. Paleontol. Mineral. Spec. Publ., 31: 293-314. Bluck, B.J., 1967. Sedimentation of beach gravels: examples from South Wales. J. Sedement. Petrol., 27: 128-156. Breed, C.S., McCauley, J.F. and Whitney, M.I., 1989. Wind erosion forms. In: D.S.G. Thomas (Editor), Arid Zone Geomorphology. Halsted Press, New York, N.Y., pp. 284307. Bryant, E.A. and McCann, S.B., 1972. A note on wind and
wave conditions in the southern part of the Gulf of St. Lawrence. Marit. Sediment., 8: 101-103. Collinson, J.D. and Thompson, D.B., 1982. Sedimentary Structures. Allen and Unwin, London, 194 pp. Cooke, R.U. and Warren, A., 1973. Geomorphology in Deserts. Univ. California Press, Berkeley, Calif., 374 pp. Mabbutt, J.A., 1977. Desert Landforms. MIT Press, Cambridge, Mass., 340 pp. McCann, S.B. and Bryant, E.A., 1972. Barrier islands, sand spits and dunes in the southern Gulf of St. Lawrence. Marit. Sediment., 8: 104-106. Nemec, W. and Steel, R.J., 1984. Alluvial and coastal conglomerates: their significant features and some comments on gravelly mass-flow deposits. In: E.H. Koster and R.J. Steel (Editors), Sedimentology of Gravels and Conglomerates. Can. Soc. Pet. Geol., Mem., 10: 1-31. Poll, H.W. van de, 1983. Geology of Prince Edward Island. Department of Energy and Forestry Report 83-1, Charlottetown, Prince Edward Island, 66 pp. Sharp, R.P., 1964. Wind-driven sand in Coachella Valley, California. Geol. Soc. Am. Bull., 75: 785-804. Sharp, R.P., 1980. Wind-driven sand in Coachella Valley, California: further data. Geol. Soc. Am. Bull., 91: 724-730. Zingg, T., 1935. Beitr~ige zur Schotteranalyse. Schweiz. Mineral. Petrogr. Mitt., 15: 39-140.