Modern carbonate-siliciclastic transitions: Humid and arid tropical examples

Abstract

Climate and tectonic setting largely control characteristics of transitions between carbonate and siliciclastic sediments in depositional systems. Three examples, selected from both humid and arid tropical settings, are discussed order to illustrate details of these relationships. The eastern shelf area of Nicaragua is an area where siliciclastic and carbonate sediments actively interfinger under humid tropical conditions. Extremely high rainfall (4.5 m yr −1 in the southern part of coastal plain), coupled with a correspondently high sediment flux to the nearshore shelf, results in an inner shelf that is dominated by siliciclastics. Density gradients and persistent momentum provided by the trade winds force turbid coastal waters to the south and confine siliciclastic sediments to a coast-parallel zone approximately 20 km wide. The remainder of the broad shelf is characterized by carbonate sediments and reefs. The mid-shelf area has a coarse carbonate sediment cover composed largely of the calcareous green alga Halimeda. The outer shelf displays rugged topography associated with reef development. Skeletal carbonates containing abundant coral and coralline algal grains are characteristic of the seaward margin of the shelf. Significant amounts of siliciclastic sediment are confined to the nearshore part of the shelf, and the transition from siliclastics to carbonates occurs over a distance of 20 km or less. A second example from the humid tropics focuses on the living reefs of Indonesia that are surrounded by fine-grained siliciclastics. A warm, rainy climate results in rapid weathering of volcanic rocks from the Indonesian archipelago and abundant sediment input to the shallow (40–70 m deep) Sunda Shelf. In the northern hemisphere winter, monsoonally driven marine processes rrive water and sediment to the east down the axis of the Java Sea. During summer the direction of transport is reversed. This bidirectional process regime not only leaves its imprint in reef and carbonate platform morphology but also results in widespread distribution of siliciclastics from adjacent source areas. Isolated reefs as well as small and generally elongate reef-rimmed carbonate platforms (0.5–10 km long) rise 40–50 m above a sea floor composed largely of siliciclastic sediments. In these cases, facies transitions are very abrupt. Until environmental conditions curtail the carbonate-producing communities of these platforms, carbonate buildups and associated skeletal sediments will coexist with wider spread and more abundant siliciclastics. A third example from a low-latitude arid setting shows that facies transitions from carbonates to siliciclastics typically take place over short distances. The lack of significant rainfall, coupled with high relief along fault-controlled basin margins, promotes aperiodic but powerful episodes of siliciclastic that build directly into the marine environment. High-sloping and narrow coastal plains composed of overlapping fans result. The steep seaward ends of these fans are maintained and protected from marine erosion by coral reefs and associated carbonates. Regionally, siliciclastic to carbonate facies changes in these arid settings take place over very shor distances, a few kilometers at most. However, on a local scale, carbonates may be replaced by siliciclastics over distances as small as a few tens of meters.