Volcaniclastic lacustrine sedimentation in the Pleistocene Guayllabamba intermontane basin in the Ecuadorian Andes

In this work, we present the description of the sedimentary fill of a well-exposed lacustrine succession in the Ecuadorian Andes. The Guayllabamba basin is an intermontane basin located in the Andean range of Ecuador, and part of its sedimentary history is represented by a volcanically-influenced ∼100 m thick lacustrine unit of the Pleistocene age. We create a stratigraphic cross-section from the eastern to western lake margins and identify nineteen facies that were used to carry out a paleoenvironmental reconstruction. The Guayllabamba paleolake was developed in a tectonic depression surrounded by volcanoes and it was filled by sediments derived from the erosion of the volcanic edifices, the reworking of unconsolidated pyroclastic deposits, and deposition of pyroclastic currents into the lake. The lake shows a deepening trend, passing from shallow deltaic sedimentation to varved diatomites with turbidites. Abundant ash-fall beds, monolithological pumiceous deltaic sequences, and pumice-dominated thick ignimbrites show the impacts of volcanism on lacustrine sedimentation within this basin. Soft-sediment deformation and gravity flow deposits are common due to the intrabasinal tectonic activity and to the intrusion of a lava body. Aulacoseira-rich diatomites dominated the background lake sedimentation. The outcrops of the Guayllabamba basin are outstanding examples of the interaction between volcaniclastic and lacustrine sedimentation.

Deltas: a new classification expanding Bates’s concepts

Deltas constitute complex depositional systems formed when a land-derived gravity-flow (carrying water and sediments) discharges into a marine or lacustrine standing body of water. However, the complexity of deltaic sedimentary environments has been oversimplified by geoscientists over the years, considering just littoral deltas as the unique possible type of delta in natural systems. Nevertheless, a rational analysis suggests that deltas can be much more complex. In fact, the characteristics of deltaic deposits will depend on a complex interplay between the bulk density of the incoming flow and the salinity of the receiving water body. This paper explores the natural conditions of deltaic sedimentation according to different density contrasts. The rational analysis of deltaic systems allows to recognize three main fields for deltaic sedimentation, corresponding to (1) hypopycnal (2) homopycnal and (3) hyperpycnal delta settings. The hypopycnal delta field represents the situation when the bulk density of the incoming flow is lower than the density of the water in the basin. According to the salinity of the receiving water body, three different types of hypopycnal littoral deltas are recognized: hypersaline littoral deltas (HSLD), marine littoral deltas (MLD), and brackish littoral deltas (BLD). The basin salinity will determine the capacity of the delta for producing effective buoyant plumes, and consequently the characteristics and extension of prodelta deposits. Homopycnal littoral deltas (HOLD) form when the density of the incoming flow is roughly similar to the density of the water in the receiving basin. This situation is typical of clean bedload-dominated rivers entering freshwater lakes. Delta front deposits are dominated by sediment avalanches. Typical fallout prodelta deposits are absent or poorly developed since no buoyant plumes are generated. Hyperpycnal deltas form when the bulk density of the incoming flow is higher than the density of the water in the receiving basin. The interaction between flow type, flow density (due to the concentration of suspended sediments) and basin salinity defines three types of deltas, corresponding to hyperpycnal littoral deltas (HLD), hyperpycnal subaqueous deltas (HSD), and hyperpycnal fan deltas (HFD). Hyperpycnal littoral deltas are low-gradient shallow-water deltas formed when dirty rivers enter into brackish or normal-salinity marine basins, typically in wave or tide-dominated epicontinental seas or brackish lakes. Hyperpycnal subaqueous deltas represent the most common type of hyperpycnal delta, with channels and lobes generated in marine and lacustrine settings during long-lasting sediment-laden river-flood discharges. Finally, hyperpycnal fan deltas are subaqueous delta systems generated on high-gradient lacustrine or marine settings by episodic high-density fluvial discharges.

LACUSTRINE DYNAMICS AND TLATEL-TYPE SETTLEMENTS FROM MIDDLE FORMATIVE TO LATE AZTEC IN THE EASTERN PART OF LAKE TEXCOCO, MEXICO

In the territories of the former lakes in the Basin of Mexico, a tlatel was an insular settlement associated with the exploitation of lacustrine resources. This study examines the stratigraphy and geomorphological context of three tlateles in the eastern part of the former Lake Texcoco and correlates their phases of development with regional paleoclimatic trends from the seventh century b.c. to the sixteenth century. The results of this research indicate that fluvio-lacustrine (i.e., deltaic) sedimentation and freshwater springs in the lake basin were important features for the establishment of tlatel settlements in the highly dynamic and saline lacustrine environment. The formation and abandonment of the studied tlateles correlate with changes in other settlements and developments recorded in other parts of Lake Texcoco. Sites and sediments in the eastern part of Lake Texcoco provide proxy information on the lacustrine changes that accompanied the development of Tenochtitlan and other lacustrine settlements in its western part.

THE IMPORTANCE OF GEOLOGICAL AND HYDROGEOLOGICAL KNOWLEDGE IN JUSTIFYING PORE PRESSURE PREDICTION: THE CASE STUDY OF THE PECIKO FIELD, LOWER KUTAI BASIN

One significant factor affecting pore pressure prediction is choosing a method of calculation. If we choose the inappropriate method, the result may not refl ect not only pore pressure condition in an area, but also geological processes operating in the whole basin. In this research, two methods are applied to wireline-based pore pressure calculation in the Peciko Field: the Eaton Method and the Soil Mechanics Method. The results of the calculation show a signifi cant difference between these methods. The Eaton Method resulted in reservoir-shale pressure discrepancy (by over-predicting the reservoir pressure) at shallower depth, and reservoir-shale pressure equilibrium at depth. On the contrary, the Soil Mechanics Method resulted in reservoir-shale pressure equilibrium at shallower depth and an under-predicting at depth. It seems that, in terms of processes operating in this fi eld which affect pore pressure regimes, i.e. lateral reservoir drainage and rapid mud-dominated deltaic sedimentation, the result of the Eaton Method is more plausible than that of the Soil Mechanics Method. This research also reveals an important inference: if there is a pressure discrepancy, then it is likely that a hydrodynamic condition is present.