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ABSTRACT: Stromatolite Morphology in Cambrian Cyclic Carbonates: Keys to Absolute Water Depth Limits and Relative Changes in Sea Level

William F. Precht, Matt H. Rostock

Platform facies of Cambrian peritidal cyclic carbonates from the Conococheague Limestone, western Maryland, are typically 2 to 6 m thick. These cycles probably reflect allocyclic responses to Milankovitch-scale (20-100 Ka) sea-level fluctuations.

Field observations coupled with previous investigations by numerous authors dearly show that morphology of stromatolites and thrombolites within individual cycles reveal absolute water depth limits on the platform interior. Cycles are characteristically represented by: (1) a transgressive surface with flat-pebble conglomerates and grainstones capped by subtidal, hemispherical, digitate, and club-shaped stromatolites and thrombolites (1-3 m depth). (2) As sea level rises over the platform, high-relief growth forms dominate in response to a marked increase in water depth (2-6 m depth). (3) Relative changes in sea level, indicative of a shallowing of the platform, are noted as these subtidal heads are often capped by algal laminates and centimeter-thick pelleted mudstones (1-4 m depth) w ich in turn are capped by cross-stratified, ooid-peloid grainstone shoals (1-3 m depth). (4) Immediately overlying these deposits are laterally linked, hemispherical-to-pustular growth forms, many of which are truncated in response to growth up to low-tide levels. (5) As accommodation space across the platform is reduced, thinly bedded stromatolites with cryptalgal laminations and low-relief convex upward features to subtidal-to-intertidal conditions dominate. (6) As relative sea level falls, laminated intertidal mudflat stromatolites are formed and are commonly bisected by prism-cracked desiccation features. (7) The overall sequence is capped by mud-cracked laminated dolomites and/or subaerial quartz-rich sands characteristic of a sabkha setting.

Although not every cycle shows the variations as described above, these morpho-types can be used to discern environmental conditions, sedimentary facies, relative cycle position, and sea-level information independently.

AAPG Search and Discovery Article #90998 AAPG Eastern Section Meeting, London, Ontario, Canada, September 10-12, 1990