A MICROMORPHOLOGICAL STUDY OF THE “BLACK MAT” DEPOSIT AT MURRAY SPRINGS AND ITS RELATION TO LATE QUATERNARY ENVIRONMENTAL CHANGES

“Black mat” deposits are organic-rich layers in Late Quaternary stratigraphic sequences widely distributed across central and western North America (Quade, 1998; Haynes, 2008; Pigati et al., 2009). The black mat at Murray Springs is believed to be an algal mat that is the stratigraphic representation of the Younger Dryas Chronozone (YD) (11-10 ka ¹⁴C BP, 12.9-11.6 ka BP) and also marks the transition from Clovis to post-Clovis cultures and the extinction of North American megafauna (Haynes, 2008). The goal of this project is to perform micromorphological analyses of the black mat at Murray Springs to determine its origins and its relation to regional climate changes during the YD. At Murray Springs, in Southern Arizona, the black mat, along with the overlying Donnet Ranch Member and underlying Sobaipuri Member were thin sectioned to determine their content and microstratigraphy. Intact blocks of sediment were taken from the exposed arroyo wall in Area 4, the multiple bison kill site. Oversize thin sections were cut from the hardened blocks and examined with a petrographic microscope using reflected light, cross-polarized light and fluorescent light. These analyses revealed the black mat lies on a distinct erosional surface devoid of in situ plant growth. It also contains fragmented plant material mixed with silt, indicating short distance transport. These observations provide evidence for flooding and inundation during the deposition of the black mat. Overall, thin section analyses indicate a sedimentary sequence of dry conditions prior to the YD, moist with ponded water during the YD and episodic moisture immediately after the YD. These results agree with previous studies (Haynes, 2007; Pigati et al., 2009) that determined the black mat formed as a regional response to Younger Dryas-related increases in precipitation and groundwater levels. Contrary to previous studies (Haynes, 2007), no algae were found in these black mat deposits. Further sampling at the Murray Springs site is necessary to determine if the lack of visible algae is due to preservation issues or lateral variation within the deposit.