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Transitional Terraced Mound to Sloping Mound Facies Relationships of a Quaternary Travertine Quarry in Belen, New Mexico


Quaternary travertine exposed in two quarries near Belen, New Mexico, was deposited as sloping tongues on the hanging wall of a fault on the western edge of the Rio Grande Rift system. The sloping nature of the paleo-topography resulted in a transitional morphology, both laterally and temporally, between terraced and sloping mounds. The travertine is interbedded with conglomerate layers and both have been cross-cut by multiple generations of fractures. The terrace mound morphology exhibits distinct pool deposits (1.5 to 2.5 m long) and rimstone dams (0.25 to 0.5 m high), in which individual layers can be traced through both features with markedly different characteristics. The pools have vertically stacked horizontal layers (0.5 to 5 cm thick) that are composed of bacterial shrubs, pisoids, rafts, and foam rock. The laterally adjacent rimstone dams are composed of thin, vertically stacked layers of ray crystal crusts that separate the pools. The location of the terrace pools and rimstone dams was fairly stationary, but through time the height of the rimstone dams decreased due to the thickening of the pools in the downstream direction. This resulted in the extinction of the rimstone dams and transition into sloping mound morphologies. The sloping mound morphology is composed of layers that dip between 5 and 20 degrees. The layers range from smooth slopes that are composed of feather dendrites to microterraced slopes with rapidly fluctuating pools and rimstone dams. The length of the microterrace pools depends heavily on the slope of the layer, with steep slopes creating small pools (2 to 10 cm) and as the slope becomes gentler the pools increase in length (up to 1 m). Regardless of length, the microterrace pools are composed of crystal fans, rafts, and pisoids. The presence of crystal fans and distinct lack of bacterial shrubs emphasizes the increasing abiotic influence for precipitation within the sloping mound morphology. This study demonstrates the transitional relationship between two distinct travertine morphologies with markedly different facies. The rapid transition between the two morphologies emphasizes the highly variable and intricate nature of travertine deposits.