Abstract

The Udden-Wentworth grain-size scale is widely used as the standard for objective description of sediment, but it inadequately covers gravel, the dominant fraction in many environments such as alluvial fans. The scale is most detailed in the sand and mud fractions, where grades such as "fine sand" are defined by particle intermediate axial length (d I ). We propose similar detailed grades for gravel with d I boundaries consistently determined by extending the Udden-Wentworth scheme of multiples of 2 (whole phi increments). The 2 to 4 mm granule class (-1 to -2 phi ) in this system consists of just one grade, but the pebble class comprises four: fine pebbles with d I from 4 to 8 mm (-2 to -3 phi), medium pebbles from 8 to 16 mm (-3 to -4 phi ), coarse pebbles from 16 to 32 mm (-4 to -5 phi), and very coarse pebbles from 32 to 64 mm (-5 to -6 phi ). Coarser grades are fine cobbles with d I from 6.4 to 12.8 cm (-6 to -7 phi ), coarse cobbles from 12.8 to 25.6 cm (-7 to -8 phi ), fine boulders from 25.6 to 51.2 cm (-8 to -9 phi ), medium boulders from 51.2 to 102.4 cm (-9 to -10 phi ), coarse boulders from 102.4 to 204.8 cm (-10 to -11 phi ), and very coarse boulders from 204.8 to 409.6 cm (-11 to -12 phi ). These terms can be used in Folk's texture classification to derive detailed descriptions such as "angular, poorly sorted, fine to coarse boulder conglomerate". This grain-size scheme is further extended to account for particles coarser than boulders (d I >4.1 m), which we collectively call mega-clasts, and the sediment they comprise megagravel or, if lithified, megaconglomerate. Megagravel is divided into four classes based on d I , including blocks from 4.1 to 65.5 m (-12 to -16 phi ), slabs from 65.5 to 1049 m (-16 to -20 phi ), monoliths from 1 to 33.6 km (-20 to -25 phi ), and megaliths from 33.6 to 1075 km (-25 to -30 phi ). The first three classes cover the coarsest sediment currently known. Their grades are fine blocks, with d I from 4.1 to 8.2 m (-12 to -13 phi ), medium blocks from 8.2 to 16.4 m (-13 to -14 phi ), coarse blocks from 16.4 to 32.8 m (-14 to -15 phi ), very coarse blocks from 32.8 to 65.5 m (-15 to -16 phi ), fine slabs from 65.5 to 131 m (-16 to -17 phi ), medium slabs from 131 to 262 m (-17 to -18 phi ), coarse slabs from 262 to 524 m (-18 to -19 phi ), very coarse slabs from 524 to 1049 m (-19 to -20 phi ), very fine monoliths from 1.0 to 2.1 km (-20 to -21 phi ), fine monoliths from 2.1 to 4.2 km (-21 to -22 phi ), medium monoliths from 4.2 to 8.4 km (-22 to -23 phi ), coarse monoliths from 8.4 to 16.8 km (-23 to -24 phi ), and very coarse monoliths from 16.8 to 33.6 km (-24 to -25 phi ). These grades also can be used in Folk's texture classification for objective sediment description. We reserve the megalith class and five attendant grades for even coarser megaclasts, with d I spanning from 33.6 to 1075.2 km (-25 to -30 phi ).

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