Facies and Morphology of Pleistocene Travertine Mounds at Lyman Lake, Arizona

Travertine mounds in eastern Arizona formed around central vents and range from <10 m to 50 m in diameter and between 0.5-60 m thick. The travertine mounds are primarily composed of laterally extensive sheets of rhythmically alternating bacterial shrubs and intraclastic layers dipping 1-3° radially away from their vent. Individual laminations of bacterial shrubs may be 0.2-2.5 cm thick, and compose layers 3-50 cm thick extending laterally for 10-70 m. Intramicrite layers 5-40 cm thick and 30-70 m long contain well-rounded to angular allochems 0.02-70 mm long. Oncoids 1-15 mm in diameter occur in lenses 3-8 cm thick and 0.5-10 m long. Carbonate rafts suspended in micrite filled pool deposits are 4-9 cm long, whereas small rafts 0.5-4 cm long occur within bacterial shrub layers. Grass molds are 1-4 mm in diameter and 1-3 cm long, whereas reeds are 0.5-2 cm in diameter and 4-15 cm long. Individual mounds contain various scales of morphological components such as vents, distal edge high angle smooth slopes, and stacked smooth slope dams. High angle slopes are concave downward structures 2-6 m long and 0.5-1.5 m thick, composed of 0.5-5 cm laminae of ray-crystal crusts that abruptly dip 30-45°. Stacked ray-crystal slopes are column 1.5-2 m tall composed of stacked layers of individual crystalline crusts 12-20 cm wide and 3-6 cm thick. These structures represent the distal edge of mounds where water flowed into the surrounding area. Vents are composed of dense ray-crystals layers that change dip dramatically as the layers transition from the vertical walls to the surrounding vent rim. Facies analysis of Mound-A indicates that the mound was deposited over six stages of deposition and composed of seven morphological features.