ABSTRACT The search for martian biosignatures can be enhanced by focusing exploration on locations most likely to contain organic-rich shales. Such shales both concentrate and preserve organic matter and are major repositories of organic geochemical biomarkers in sediments of all ages on Earth. Moreover, it has been suggested that for Mars, accumulations of organic matter may be the most easily detected and least ambiguous of possible biosignatures ( Summons et al. 2010 ). Since current surface conditions on Mars are unfavorable for preservation of organic matter, focusing exploration on locations predicted to contain ancient organic-rich shales would offer one of the best chances of detecting evidence of life—if it ever evolved on the planet. Orbital data can be used to evaluate regional sediment sources and sinks on Mars, and, based on that, facies can be predicted and locations identified that are most likely to contain organic-rich sediments. An example is presented from Acidalia Planitia, in the martian lowlands, where this approach led to the conclusion that facies in southern Acidalia were likely to be dominated by fine-grained, muddy sediments. That conclusion added weight to the hypothesis that mounds in Acidalia are martian versions of mud volcanoes as well as the suggestion that organic materials, if present, would have been deposited in the same area as the mounds. This allowed speculation that potential mud volcano clasts in Acidalia could include preserved, organic biosignatures and, thus, that the mounds in Acidalia constitute an untested class of exploration target for Mars. Facies prediction using orbital data is particularly applicable to planetary exploration where ground truth is most often lacking but orbital data sets are increasingly available. This approach is well suited to the search for potential geochemical biomarkers in organic-rich shales. The approach additionally could be applied to exploration for other categories of biosignatures (such as stromatolites or morphologically preserved microfossils) and to more general planetary objectives, such as the search for hydrothermal sediments, carbonates, or any particular type of geologic deposit.