QUANTIFYING THE ROLE OF DIATOMS ON THE POROSITY OF CARBONATE ONCOIDS FROM A NORTHERN CALIFORNIA SPRING: IMPLICATIONS FOR SHELLY FAUNA-ASSOCIATED POROSITY IN TERRESTRIAL CARBONATES

Terrestrial spring-associated carbonates produce various sedimentary products often with laminated and porous textures that are controlled by water flow, detrital grains, water chemistry, temperature, and biology. Factors that govern lamination continuity can influence the paleoenvironmental reconstruction potential of terrestrial carbonates as well as their primary porosity. We investigate the effect of diatoms on the textural variability and porosity of coated grains (oncoids) from a freshwater spring in Henry Cowell State Park, northern California. Oncoids (1-2 cm in diameter) accrete along an ~800 m transect and accumulate in pool carbonate facies along the flow path. Sixty oncoids were collected from the distal-most pool facies of the spring (~ 300 m from source) and the porosity of the 1 cm diameter fraction was determined using the ImageJ software on thin section grain mounts by adjusting the threshold size for pores greater than 1000 microns². Results reveal a positive, statistically significant relationship between the number of pores and the number of diatoms (R²= 0.58). The pores that contain diatoms are associated with a larger pore area than the pores that do not have diatoms. There is a positive relationship between the minor axis diameter of the pore and the diameter of the largest diatom in a pore (R² = 0.36). The laminated textures contain fewer diatoms than the porous textures suggesting that the diatoms can alter lamination continuity. These findings have implications for the influence of shelly fauna on the syndepositional and diagenetic porosity of carbonate rocks.