Amalgamated Interdune Deposits, White Sands, New Mexico

Abstract

ABSTRACT Cores taken by McKee and Moiola (1975) from White Sands dune field indicate that interdune areas are underlain by 7.0-10.4 m of clastic gypsum deposits. Clean sand bodies as much as 4.5 m thick are separated by thin beds of silly sand. McKee and Moiola interpreted the sequence to be the product of climbing bed forms; interdune silty sands were interpreted to be deposited upon truncated dune sands and to be soon buried by the next wave of advancing dunes, If the dunes are climbing in such a manner, a pinchout of interdune sediments downwind across interdune surfaces is required. The thickness of the silly sands should be related to the size of the interdune area and the rate of dune migration. We tested this autocyclic hypothesis by trenching the length of the interdune areas in both t e barchanoid and transverse dunes. Trenches revealed that interdune deposits form flat-lying sheets which extend uninterrupted to positions directly beneath the windward slopes of dunes at the downwind end of each interdune area. This relationship precludes the possibility that, under present conditions, thick dune sands are being preserved. Because there is little or no net accumulation of dune sand, interdune deposits are being amalgamated; thin lenses of dune sediment exposed on the wall of one trench show that a minimum of three interdune deposits have been fused to form the uppermost 30 cm of the interdune deposit. Clean sand in the cores may have accumulated during periods when sediment supply was greater than at present. Silly sediments accumulated during periods when, as in the p esent, sand supply was limited and dunes did not climb appreciably. Future studies of modern and ancient eolian sediments should attempt to document whether the interdune deposits represent deposition within single interdune areas or amalgamation of several discrete deposits.