GROWTH OF JURASSIC BELEMNOID ROSTRA FROM NORTHEASTERN UTAH: INSIGHTS USING CATHODOLUMINESCENCE

Standard thin sections of fragmental Jurassic belemnoid rostra, collected from the Curtis Formation, eastern Uinta Mountains, Utah, were examined using petrographic methods and cathodoluminescence. Microstructure of the rostra consists of radially arranged, nonferroan, calcite crystals traversed by numerous growth lamellae that become more closely spaced toward the rostrum's outer margin. The rostral crystals are syntaxial across growth lamellae. Under cathodoluminescence, most rostral crystals are dully luminescent. In contrast, some growth lamellae, infilled microfractures, and the cement of the material infilling the alveolus are brightly luminescent. Scanning electron microscopy of etched specimens demonstrates the continuity of the growth lamellae as discrete platy carbonate microlaminae suggesting that the brightly luminescent material is actually a replacement product of the lamellae. However, the relationships are not totally clear because some growth lamellae do not luminesce and there are brightly luminescent bands that do not correspond to obvious growth lamellae when viewed in plane polarized light. Belemnoid life span is an unsettled question. We believe that crowding of growth lamellae and the concentration of diagenetic carbonate along their surfaces suggests the possibility that significant interruptions to growth occur along those surfaces. A rapid to slow growth history, with occasional interruptions, is consistent with a long, perhaps continuous, growth history for at least some belemnoids.