TAPHONOMIC AND SEDIMENTOLOGIC IMPLICATIONS OF CRINOID INTRASKELETAL POROSITY

The porous texture of crinoid columnals has a number of implications: (1) After soft tissue decays, the originally articulated skeleton of t he crinoid falls apart. (2) The porous ossicles become part of the sed iment, but they will behave hydrodynamically like much smaller solid m ineral grains. (3) Because of their hydrodynamic behavior, crinoid pla tes should be transported away from their life habitat with relative e ase. (4) The porous skeleton should act as a 'sponge' during diagenesi s and serve as the site of deposition of diagenetic minerals. Porosity and specific gravity of columnals from five species of modern, deep-s ea crinoids and from as many as seven form-species of Carboniferous cr inoids were determined from either back-scatter images from an electro n probe or from the volume and mass of individual columnals (extant sp ecies only). These approaches give porosity values of 52-72% for moder n crinoids and 39-69% for fossil crinoids. This corresponds to net spe cific gravities of 1.47-1.83 for modern columnals and 1.54-2.10 for fo ssils (solid calcite has a specific gravity of 2.72). In order to comp are hydrodynamic behavior of crinoid plates with more dense mineral gr ains, settling rates and flume entrainment thresholds were determined for columnals of the five modern species. Columnals have settling rate s and entrainment thresholds equivalent to quartz spheres with diamete rs less than a tenth of the diameter of the columnal (measured as larg est dimension). A columnal's unit immersed mass (UIM: effective mass, a function of specific gravity, divided by the projected area of the c olumnal) correlates fairly well with settling and entrainment velociti es; however, columnal shape significantly influences behavior. Grain-s ize data from cross-bedded, crinoid-bearing grainstones and quartz are nites show this differential behavior between crinoid and other grains . If intraskeletal cementation predates burial, a columnal's specific gravity would be increased, affecting its hydrodynamic behavior. There fore, both the timing of organic decay and diagenetic filling of poros ity relative to final burial should influence the transport history of a columnal.