The Marjum Formation (~400 m thick) of the House Range, western Utah was deposited in the axis of an embayment that extended inland into western North America during Middle Cambrian time. Three large-scale sequences or “Grand Cycles” (~180 m thick) are recognized and are composed of shaley limestone facies grading into clean limestone facies. The focus of this study is the limestone-dominated portion of the upper Grand Cycles (upper Marjum Formation). The upper Marjum Formation (~100 m thick) is characterized by nine cycles (4-30 m thick) composed of thin, rhythmically interbedded limestone and marl layers (rhythmites) overlained by thin stromatactis-bearing mud mounds. Rhythmite limestone layers are composed of dark, laminated pelletal lime mudstone (~3 cm thick). Marl layers (~2 cm thick) are composed of laminated, argillaceous, dolomitized, pelletal lime mudstone; both lithologies contain rare agnostid trilobites and sponge spicules. Mud mounds are composed of bioturbated pelletal lime mudstone with trilobite wacke/packstone lenses, and spar-filled stromatactis structures. The mounds are between 0.4-1.5 meters thick and approximately I meter in diameter. The mound flanks thin to bioturbated layers which connect to adjacent mounds over distances of ~ 0.5-2 meters. The stromatactis structures vary in size and shape from mm wide and 4 cm long to 7 cm tall and 75 cm long and are geopetally filled with pelletal sediment and two generations of cement. The stromatactis structures parallel bedding planes and become significantly larger in the mound cores.

Preliminary interpretations suggest that rhythmites were deposited in anoxic waters below storm wave base, based on the lack of current reworked features, preserved suspension laminae, paucity of benthic fossils, and the lateral continuity of individual beds. The mud mounds are also interpreted as deep-water deposits based on the lack of change in grain size from underlying rhythmites, lack of photosynthesizing organisms, and lacked of current reworked features. However, the abundance of bioturbation in mud mounds indicates more oxic bottom waters.

The cyclic change from laminated rhythmites (non-bioturbated) to bioturbated mud mounds suggests bottom-water oxygen level fluctuations. Changes in bottom-water oxygen levels could be due to: 1) wet to dry climate cycles affecting fluvial-derived nutrient fluxes, 2) changes in wind direction or intensity, which would affect upwelling patterns, and/or 3) surface-water temperature changes influencing primary productivity. Geochemical analysis including carbon isotopes, TOC, insoluble residues are being conducted to help determine what factor(s) influenced the observed bottom-water oxygen level changes.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah