Sedimentology and stratigraphy of the Triassic Chinle Formation, eastern San Juan Basin, New Mexico
Abstract
Upper Triassic continental ëtrata of the Chinle Formation, in the Nacimiento Mountains, New Mexico, represent two cycles of fluvial to fluvial-deltaic to marshland-lacustrine sedimentation. Each cycle was initiated by uplift in the southern Ancestral Rocky Mountains and deposited on the local palaeoslope. However, the influence of this highland on sediment transport and supply diminished with time. Depositional trends exhibited up-section are: 1) fluvial regime changed from a high energy braided stream to a very low energy meandering stream system; 2) palaeotransport directions shifted from southwestwardly to north-northwestwardly; 3) the initial crystalline and metasedimentary source terrain was gradually replaced by a widespread sedimentary one; and 4) mean and maximum grain size decreased. Decreases in fluvatile energy and grain size upwards reflect the lowering of uplands by erosion and the evolution of a continental drainage system on a gentle gradient. Also, the uplift which generated the lower fluvial sequence probably had a steeper initial palaeoslope and was of greater magnitude than the second uplift. Shifts in palaeotransport and provenance are not due to active tectonism, but rather, are a passive response to the waning influence of local highlands on regional sedimentation patterns. The locations and distributions of sedimentary copper deposits, found in the fluvial channel sequences of Chinle sandstones, are chiefly dependent upon the shape and porosity of the channel deposits,and the amount of organic matter that was present to reduce copper-bearing fluids. Porosity of these channels, as it affects copper mineralization is controlled by the amount and' time of formation of calcareous cement.
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