Ignimbrites of Amazonis Planitia Region of Mars

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Abstract
A series of postulated ignimbrite units is mapped in the Amazonis, Memnonia, and Aeolis quadrangles of Mars. The units cover about 2.2×106 km2 within a broad but discontinuous and irregular belt trending east‐west along the highland‐lowland boundary. The ignimbrites overlie parts of the western and southern aureole materials of Olympus Mons but are embayed in places by the lava plains of the lowlands. Stratigraphic relations between the basalt flows from the Tharsis Montes region and the ignimbrites are not clearly defined; crater counts suggest that the younger ignimbrites postdate the lava flows. Crater counts per square kilometer for the ignimbrites range from 7.29±1.95×10−4 to 6.36±2.01×10−5 for craters larger than 1 km in diameter. The ignimbrite materials form thick (≥100 m), extensive, relatively flat sheets that are smooth to grooved or gently undulating. Grooved surfaces appear to be yardangs and, in most places, are not alined with prevailing wind directions. The seven mapped ignimbrite units are characterized by morphologic expression, stratigraphic position, and crater counts. Similarities to ignimbrites in the Pancake Range of central Nevada include (1) rounded patches of smooth, high‐albedo, nonwelded material superposed on jointed, low‐albedo, welded material, (2) local complementary joint sets in welded materials, and (3) thick flow sheets of great areal extent that follow but subdue underlying topography. Four major eruptive centers occur in areas where units are thickest and where a dominant, NNW‐SSE structural trend is expressed locally by unit margins, elongate collapse features, and normal faulting. A minimum volume of 3.85×106 km3 for the deposits has been calculated from thickness estimates based on shadow measurements and crater rim height relations.