Coal petrographic genetic units in deltaic-plain deposits of the Campanian Mesa Verde Group (New Mexico, USA)
The Campanian rocks of the Mesa Verde Group in the San Juan Basin, New Mexico, USA, form a littoral wedge composed of offshore, shoreface and onshore facies. This siliciclastic wedge was interpreted as a landward-to-seaward stepping of grouped genetic units, i.e. intermediate term cycles. The continental facies consist of coastal-plain deposits (coals, shales, and sandstones) represented by three main domains, progressively distant from the palaeoshoreline: the deltaic-plain, the intermediate-plain, and the alluvial-plain domains. In the deltaic-plain, grouped aggradational–retrogradational genetic units consist of preserved coal-shale doublets alternating with thick shales. Although previous works have identified genetic units in the shoreface facies, the existence of the latter has not been proven in the coastal-plain deposits. As coal-shale doublets may represent such genetic units, an organic petrography study was performed at high resolution on an 80-cm thick, 1.5-km long coal seam to demonstrate this hypothesis. In this work, the depositional environments of the initial peatlands were rebuilt utilizing two complementary methods: maceral and microlithotype analyses. The results reveal the existence of internal sequences in the coal seam, corresponding to recurrent patterns of a palaeogeographical model proposed for a littoral domain. The authors interpret this coal sequence as the record of successive progradational and aggradational events. Furthermore, a previous study of the organic content of the shales located above the coal demonstrated that the preservation of these facies is related to higher accommodation/peat production and accommodation/sedimentary supply ratios. Coal-shale doublets are then interpreted as elemental genetic units of the deltaic-plain formation. This study demonstrates that, contrary to original concepts, littoral coal seams are not just markers of stable shoreline conditions at a high time scale, but also record landscape migration.