Sedimentologic comparison of the late/lower early middle Cambrian Altona Formation and the Lower Cambrian Monkton Formation.

摘要

The Altona Formation represents the oldest Cambrian sedimentary unit in northern New York, recording cyclic deposition in shallow marine and fluvial environments under both fair-weather and storm conditions. Five outcrops and one well log were measured and described at the centimeter scale and the top and bottom contacts of the Altona were identified. Based on the recognition of sedimentary structures such as hummocky cross stratification, oscillatory ripples, graded bedding, trough and tabular cross stratification, and bioturbation, as well as subtle lithologic changes, six lithofacies representing non-marine, middle to upper shoreface, offshore, and carbonate ramp environments were identified. The top contact with the overlying Ausable Formation is characterized by inter-tonguing marine to non-marine siltstones and cross stratified medium sandstones. The lowermost Altona is found to lie only one meter above Precambrian basement and is interpreted to be the only non-marine facies in this unit. Throughout the 84-meter thick section, stratigraphy records a transition from upper/middle shoreface to carbonate ramp deposition and offshore muds before cycling between upper shoreface, carbonate ramp and non-marine deposits. Based on parasequence architecture, this section of rock is interpreted to represent the transition from the transgressive systems tract to the highstand systems tract.;Thin sections analysis from each lithofacies quantified grain size and composition and identified a provenance. Modal analysis data from clastic lithofacies reveals subarkose to arkose sandstones with an accessory mineral suite including ilmenite, apatite, rutile, and zircon. Integrating the compositional data, particularly the accessory mineral suite, with detrital zircon dates of 1000 -- 1300 Ma (Chiarenzelli et al., 2010) suggests that the Grenville Adirondacks in particular the AMCG suit and Lyon Mountain Granite are a likely source rock.;Comparison with the Monkton Formations of Vermont suggest that these two units were deposited under similar sea level conditions and are therefore correlative. Provenance study suggests that they were both sourced form the Adirondack Mountains. The major difference is in their depositional environments as the Monkton represents deposition of predominantly tidally influenced deltaic environment. The environmental processes acting on the two units suggests that the paleogeography of the Iapetus margin in this area was an embayed coastline.