IMPACT INTO UNCONSOLIDATED, WATER-RICH SEDIMENTS AT THE MARQUEZ DOME,TEXAS

The Marquez Dome, Leon County, Texas represents a 13 km diameter Paleo cene/Eocene impact structure formed in largely unconsolidated sediment s in a near-shore environment. The present study is an analysis of sam ples from cores taken from boreholes drilled separately on the edge of the central uplift and in the surrounding annular basin. The borehole drilled in the annular basin of the structure penetrated a sequence o f interbedded sands, silts, and shales that is typical of the stratigr aphy of the surrounding area. In contrast, the borehole drilled on the edge of the central uplift penetrated material that is relatively hom ogeneous in chemical composition and texture and may represent a mixtu re of sand, silt, clay, and minor carbonate derived from deeper levels in the preimpact stratigraphy. Veins containing pseudotachylitic brec cias are not found and are not expected in this environment because lo w-strength target materials are not conducive to frictional melting. S imilarly, the low strength and unconsolidated nature of these target m aterials are not conducive to the formation of other types of typical impact breccias (e.g., melt rocks or suevites). The absence of such li thologies results either from explosive ejection of these materials ca used by the water-rich character of the target sediments or, more prob ably, from removal of these materials by deeper postimpact erosion tha n has been suggested previously. Planar deformation features (PDFs) we re not found in quartz grains from any of these samples. The scarcity of quartz grains with PDFs, which have only been reported in rare impa ct breccias from the central uplift, and the large amount of vertical displacement indicated for the central uplift of this structure may al so be a consequence of the low strength of target materials.