Karst Landforms in the Lagoa Santa Area

Abstract

The karst landscape in the Lagoa Santa area is characterized by a full suite of typical karst landforms, with some peculiar features commonly not found in other karst areas. The karst is developed in Proterozoic carbonates of the Sete Lagoas Formation. The pure upper Lagoa Santa Member, which covers 44% of the area, displays the majority of the karst features, while the impure lower Pedro Leopoldo Member outcrops at the borders and exhibits subdued karst landforms. The Lagoa Santa Karst is mostly a covered karst, with only 11.5% of the total area presenting karstified outcrops, which are more frequent in the dissected areas toward the east. Water recharge is predominantly autogenic, as the region displays limited noncarbonate catchment areas, with drainages directed toward the Velhas River, the main base level, and Mata Creek. Seven geomorphological domains can be recognized in the area, with karst landforms being more developed in the karst plateaus. Due to its covered nature, many karst landforms, such as karren, towers, and corridors, initially developed under the soil and were gradually exposed at the surface. Karst landforms thus exhibit a strong inheritance of subsoil processes. The most common karren type is the joint karren, which developed along subhorizontal bedding planes and joints and formed initially under the soil, with a later evolution at the surface. Typical karst landforms occur, such as various karren types, dolines, dry valleys, swallets and springs. In addition to these karst forms, the area is characterized by peculiar limestone cliffs, which represent sharp vertical breaks in an otherwise undulating soil-covered landscape. These cliffs tend to present swallets or dolines at their bases, causing progressive lowering at the base and increasing cliff exposure. Large depressions mostly occur close to the base levels. The unusual occurrence of lakes is also noteworthy. Three types of lakes can be recognized: (i) Sinkhole lakes formed due to the plugging of swallets by impermeable soil-derived material, (ii) epikarst lakes in the lower portions of the area and (iii) lakes over metapelite. The Lagoa Santa Karst has experienced a long evolutionary history dating back to the period when the upper carbonate sequences were exposed, probably well within the Tertiary, assuming regional uplift rates. The karst shows strong control both in the tectonic framework and in the direction of the hydraulic gradient toward the NE. Depressions, groundwater flow, and other karst features show well-marked alignment, especially when the joint direction, bedding dip, and hydraulic gradient are concordant.