What is Boundstone?

Boundstone is an indigenous deposit of limestone that was bound by algae, coral, or other unicellular organism when it was formed. Boundstone is found in areas surrounding coral reefs, and areas that were coral reefs 2.5-3 million years ago, but may be surrounded by dry land today. Depending upon the way the organic matter was arranged within the sediment when the stone was formed and what kind of organic matter it was, boundstone can be classified as either framestone, bindstone, or bafflestone.

Framestone is distinguished by a rigid framework of fossilized organic matter, usually deep-water sponges, that occur close together in a ”life position.” These sponges were bound by microbial crusts and hardened sand when living, and the spaces between gradually filled with sand, sediment, and calcite crystals. Over time, as the water receded and the structure was continuously exposed to air, the natural cementing action of the densely packed sediment preserved the remains of the organic matter as fossils. Framestone is named for the “framework” the organic matter provides, and can be found in massively large or small, dense deposits.

Bindstone is characterized by a more random dispersion of organic binding agents that may or may not be preserved. The most common binding agent in bindstone is algae, which holds together layers of mud and calcite with large “pores” caused by gas bubbles that became trapped in the sediment during formation. Stromatolites, which are fossilized mounds of layered algal mat and sediment, are the most common form of bindstone. Most bindstone deposits are oriented vertically as opposed to horizontally, due to the layering manner in which they form. The base of the stone is formed as an algal mat forms laterally, then is built upon by mud and other algae both vertically and laterally, changing with environmental conditions. Bindstone is the most commonly found type of boundstone.

Bafflestone is bound by thick-walled, parallel columns of coral, which permit only finer-grained sediment to pass through. As a result, the composition of bafflestone, other than fossilized coral, is mostly naturally-cemented sand and silt. The sand is composed of homogenous calcite and the silt consists of a mixed residue left behind after carbonate mud was filtered out. The unique structure of bafflestone means that it can only form on and around colonies of vertically-growing coral, and are therefore limited to small individual or clustered deposits. Bafflestone is named for the “baffling” action exhibited by the coral columns during the formation of the stone.