LITTORAL DRIPSTONE AND FLOWSTONE - SPELEOTHEM-LIKE PRECIPITATES ON A TROPICAL CARBONATE COAST, TINIAN AND ROTA, MARIANA ISLANDS

Speleothem-like deposits have been observed in uplifted Holocene marine notches and coastal overhangs on Tinian and Rota, Mariana Islands. Termed “littoral dripstone” and “littoral flowstone” to distinguish them from true cave deposits, they reflect the basic speleothem types: draperies, stalactites, stalagmites and coralliform patches. They lack the luster and crystallinity of cave analogues and are not as well-developed, dense or massive. They are composed of highly porous layered microcrystalline calcite and aragonite with inclusions of marine calcareous grains. Actively dripping deposits are commonly observed and their formation is apparently associated with both groundwater seepage and sea water spray.

They are found in highly specialized environments that are regularly, but indirectly wetted by sea spray and waves, with distribution limited both horizontally and vertically. In coastal alcoves and areas partially protected from surf by reefs and beach deposits, they are found 2-3 meters above sea level, but where wave action is more vigorous they are present only in more elevated notches, 5-6 meters above sea level. These deposits appear to be limited in time: lacking sturdiness, they are removed on a routine basis by intense storm events. Growing back quickly and being episodically stripped away is compatible with their location and the lack of many big specimens.

Aside from providing valuable insights into the previously unrecognized carbonate deposition in the modern supratidal zone on tropical coasts, these littoral, non-spelean vadose precipitates are of practical interest to geomorphologists. If not properly interpreted, they can cause misinterpretation of wave cut or bioeroded landforms as remnants of solution cavities; therefore the distinction between true speleothems, indicators of karst paleoenvironments, and these littoral precipitates, contemporary coastal landforms, is crucial.