Sedimentary fades and stratigraphic architecture in coarse-grained deltas: Anatomy of the Cenozoic Camana Formation, southern Peru (16°25′S to 17°15′S)

摘要

In the external forearc of southern Peru (Arequipa region), the sedimentary facies and the stratigraphic architecture of the Cenozoic Camana Formation are presented in the context of tectono-eustatic controls. The Camana Formation is defined as ～ 500 m thick coarse-grained deltaic complex that accumulated in a fault-bounded elongated depression extending from the Coastal Cordillera in the east to the offshore Mollendo Basin in the west and likely up to the Peruvian Trench. Based on the analysis of facies associations, we propose a refined stratigraphic scheme of the Camana Basin fill. The Camana Formation was formerly divided into the Camana "A" and Camana "B" units (CamA and CamB, respectively). We reinterpret the stratigraphic position and the timing of the CamA to CamB boundary, and define three sub-units for CamA, i.e. sub-units A1, A2, and A3. Each depositional unit shows individual stacking patterns, which are linked with particular shoreline trajectories through time. Strata of Al form the basal succession of the Camana Formation and consist of distributary channels and mouth bars, unconformably overlain by beds of A2. A2 consists of delta front deposits arranged in voluminous clinothems that reflect a progradational downstepping complex. A3 consists of delta front sandstones to prodelta siltstones arranged in retrogradational onlapping geometry. A pebbly intercalation in proximal onlapping A3 deposits is interpreted to reflect pulses of uplift in the hinterland. The overlying CamB unit is characterized by a thick alternation of fluvio-deltaic conglomerates and sand bars. The ages of the individual units of the Camana Formation are not yet well defined. Based on the available information and stratigraphic correlations we tentatively assign A1 to the Late Oligocene, A2 to the Early Miocene, A3 to the late Early Miocene to early Middle Miocene, and CamB to the Late Miocene to? early Pliocene. The sub-units A1 and A2 represent a regressive systems tract, where the shoreline was forced to migrate seaward. This scenario differs from the Early Miocene eustatic sea-level rise suggesting that significant tectonic uplift along the Coastal Cordillera controlled the high sediment influx during A2 deposition. The sub-unit A3 represents a transgressive systems tract, triggering landward migration of the shoreline. This scenario is well in line with the global sea-level chart suggesting that A3 has been deposited during a phase of eustatic sea-level rise with minor tectonic activity. The fluvial deposits of CamB reflect an increased sediment flux due to uplift of the hinterland. The observed stratigraphic patterns support predominant tectonic control on sedimentation in the Camana Basin and the established stratigraphic framework provides an essential baseline for future correlations of the Cenozoic sedimentation in the forearc area of the Central Andes.