Iron-stained-quartz as record of recent reworking of older sediment by natural and anthropogenic processes, Rio Grande Delta, Texas

The iron-coated quartz grain record in the Rio Grande deltaic plain in sout hern Texas, determined from petrologic study of surface and core samples, d oes not show the distribution pattern typically observed in other deltas. T he iron-stained grain distribution patterns in this delta are highly irregu lar in both time and space, and do not display low proportions (< 10%) of c oated grains, in surficial and Holocene subsurface deltaic sections versus distinctly higher proportions (to over >20%) of stained grains in underlyin g Pleistocene alluvial strata. Intermediate (>10%) to large (>20%) amounts of partially plus fully coated quartz particles are measured in most surfic ial samples of 10 sampled deltaic depositional environments and in Holocene core sections. This is a result of (1) pre-dam fluvial transport of older, iron-stained material from Rio Grande basin areas to the Holocene cover of the delta and its Gulf of Mexico margin, (2) erosion and recent reworking- landward of iron-stained sediment from Rio Grande delta terrains now submer ged on the inner and mid-shelf, (3) possible in situ formation of iron-coat ed grains in the lower valley and delta proper, and (4) intensified post-de positional reworking of the deltaic plain and upper Holocene sections by hu man activity during the past century. We suggest that anthropogenic activit y has now replaced natural processes, especially fluvial transport, as the primary means of mixing older material onto the younger surficial. Rio Gran de deltaic plain. The delta's present sediment cover is interpreted as a 'p alimpsest' comprising admixtures of reworked modem and relict material, par ticularly in areas where formerly buried sediment continues to be artificia lly exposed and modified. We anticipate that the proportion of iron-coated grains will remain high at the Rio Grande deltaic surface as human activity continues to replace natural fluvial transport as the dominant process.