ABSTRACT

Sandstone compositional data can be a powerful tool in the interpretation of tectonic and climatologic influences on sedimentary basin fill, in addition to yielding important information about porosity in sandstone petroleum reservoirs. In order to explore these relationships, a modified Gazzi-Dickinson point-counting technique was used to analyze the composition of 143 Mesozoic sandstone samples from the southern Junggar, northern Tarim, and western Turpan basins of northwestern China. Results indicate that a Mesozoic, ancestral version of the Tian Shan physiographically separated the Junggar and Tarim basins and provided sand of very different composition to each basin. Mesozoic sandstone from the northern Tarim basin is diverse in composition and lithic-rich (Qm₄₁F₁₄L₄₅; Qp₄₂Lvm₂₆Lsm₃₂), is locally micaceous, and contains common radiolarian-chert grains and few dense accessory minerals. Inferred source rocks include upper Paleozoic alkali granite and metamorphic complexes, thick Silurian bedded-chert sequences, and lower Paleozoic strata of a passive continental margin. In contrast, sandstone from the southern Junggar and western Turpan basins is uniformly volcanic-rich (Qm₂₁F₂₁Lt₅₈; Qp₁₃Lvm₆₈Lsm₁₉), and contains abundant dense accessory minerals and only local radiolarian chert and mica. Inferred principal source rocks are Devonian-Carboniferous andesitic arc volcanics.

The effect of sampling scale on sandstone composition outweighs that of plate-tectonic setting. Samples were derived mostly from medium- and coarse-grained fluvial systems that likely drained only portions of the ancestral Tian Shan and hence preserve local source-rock signatures, rather than an integrated compositional signal that can be directly compared to plate-tectonic petrofacies models. In addition, though Mesozoic basins of western China were most akin to broken foreland basins, Mesozoic sandstone is considerably more compositionally diverse and lithic-rich than that of modern or ancient broken foreland basins because of the variety of accreted terranes constituting the ancestral Tian Shan.

Temporal changes in sandstone composition are consistent with episodes of Mesozoic deformation in the Tian Shan. Each deformational episode increased physiographic relief of the ancestral range, produced renewed downcutting and erosion of source rocks, and resulted in the deposition of compositionally very immature sandstone in adjacent basins. Although a regional early Mesozoic megamonsoon and an Early Cretaceous rain shadow cast across the northern Tarim basin are interpreted from regional facies and paleontologic data, neither paleoclimatic phenomenon appears to have significantly modified sandstone composition in the study area.

Calculations of intergranular volume (% porosity + % cement) indicate that porosity in sandstone from the Tarim and Junggar basin depocenters was reduced principally by burial compaction and that the rate of porosity reduction was highest for lithic-rich samples.