Sedimentological Response of an Alluvial System to Source Area Tectonism: The Seilao Member of the Late Cretaceous to Eocene Purilactis Formation of Northern Chile

The Seilao Member of the Late Cretaceous to Eocene Purilactis Formation in northern Chile consists of alluvial fan deposits organized into a series of large-scale (≥100m), basin-wide, coarsening and fining upwards sequences (megasequences). Coarsening upwards sequences record a change from playa and sandflat sedimentation to poorly confined conglomeratic sheetflood and hyperconcentrated flood flow deposits and show a decrease in incision up section. Conversely, fining upwards megasequences are the reverse of this sequence. Fining upwards megasequences may be overlain by thick sequences dominated by small-scale (3-20 m) fining upwards cycles composed of pebbly braided stream deposits. Megasequences developed in response to tectonism. Coarsening upwards cycles resulted from the easterly progradation of gravels during limited source area uplift/relative basin subsidence. Relative subsidence was sufficient to permit aggradation in proximal areas (with no incision) and gravel progradation in more distal areas of the basin. Fining upwards megasequences developed during periods of rapid source area uplift/relative basin subsidence. Small-scale fining upwards cycles result from periods when sediment supply and subsidence were close to equilibrium; consequently, allocyclic influence on alluvial architecture was minimal and autocyclic processes dominated. Although difficult to constrain, the Late Cretaceous tectonic setting in the north Chilean Precordillera suggests that source area uplift resulting in relative basin subsidence was the most important control on sedimentation. Uplift resulted from inversion along pre-existing extensional faults (possibly associated with the development of an easterly propagating thrust front) following mid-Cretaceous compression (Peruvian Orogeny). Alternative allocyclic mechanisms had only a minimal influence on alluvial architecture. In particular, no change in macroclimate or source area composition took place during sedimentation and eustatic fluctuations were unimportant in this closed, intermontane basin.