Karren features in Island Karst: Guam, Mariana Islands

摘要

Dissolutional sculpturing (karren) in island karst terrain is distinct from karren in inland continental settings, whether temperate or tropical. Reef, lagoonal and eolian limestones that form most young carbonate islands are eogenetic, meaning they have not undergone significant diagenesis and exhibit high primary porosity and extreme heterogeneity. These lithologic qualities, combined with other characteristics of island karst, including the effects of autogenic recharge, tropical climate, and the proximity of the ocean, result in the development of unique karren forms. Highly irregular, composite karren forms are dominant, while linear forms, especially hydrodynamically shaped features, are rare or absent.The most common karren type on Guam is an assemblage of densely packed solution pits, separated by jagged ridges and sharp tips. It dominates the surfaces of all young reef limestones and ranges in texture from extremely jagged coastal forms, to somewhat more subdued inland features. It covers large areas, forming karrenfelds of jagged pit and pinnacle topography. Lacking a unique and accurate geomorphic term, this karren assemblage exists in a variety of similar forms, and its development is poorly understood. We propose the term "eogenetic karren," as it emphasizes the eogenetic nature of host limestone as the common factor controlling the development of variants of this karren type, while avoiding references to geographic settings or any of the poorly understood and variable genetic mechanisms.In addition to eogenetic karren, other forms of karren occur on carbonate islands but are limited to specific lithologic and environmental settings. Hydrodynamically-controlled features, dominant in interior continental settings of both classical temperate and tropical karsts, are nearly absent on Guam and similar islands, and form only locally in outcrops of dense, diagenetically mature, and recrystallized, limestones.