Paleoglobal change during deposition of cyclothems: calculating the contributions of tectonic subsidence, glacial eustasy and long-term climate influences on Pennsylvanian sea-level change

Abstract

New sedimentological determinations of the water depth and associated sea-level change of Midcontinent Pennsylvanian cyclothems shows that they accumulated in water depths ranging from as low as 32 m to as high as 160 m. depending on which model is used to establish the deepest water facies. These depth determinations also indicate that regardless of model, depth variations existed for different cyclothems both laterally and in time. Average water depth determinations and sea-level change for models of Heckel and Gerhard are 96.4 and 86.0 m, respectively. Analysis of tectonic subsidence permits calculation of the magnitude of tectonic processes and associated climatic effects, which controlled changes in sea level during deposition of Pennsylvanian cyclothems. Far-field tectonic effects in response to regional orogenic movements partially influenced Pennsylvanian sea-level change in the Midcontinent. Organization of Virgilian and Missourian mid-coninent cyclothems into four- to five-fold bundles suggests that sea-level changes in Midcontinent platform areas were influenced both by Milankovitch orbital parameters and longer-term climate change, whereas Desmoinesian sea-level change apparently was influenced more strongly by tectonic subsidence controlled by foreland-basin tectonism. Sea-level change associated with cyclothems in the Illinois basin, Central Appalachian basin, and the Asturias basin of Spain were controlled mostly by foreland basin subsidence and later strike-slip faulting. Sea-level change in the southeast Netherlands coal field was influenced primarily by strike-slip movements within the Hercynian tectonic orogenic zone. The magnitude of tectonically contributed change in sea level varied laterally. In the Midcontinent, tectonic subsidence accounts for ~ 5–20% of the total sea-level change in platform areas, and perhaps as much as 20% in basin depocenters, whereas in the Illinois basin, tectonic subsidence accounts for 90% of estimated sea-level change. The remaining change in sea level is controlled by both short-term glacial eustasy (Milankovitch orbital forcing; approximately up to 70% of sea-level change in the Midcontinent) and long-term climate change (~ 15% of sea-level change in the Midcontinent). These findings suggest that away from orogenic belts, climatic change is the principal driving mechanism controlling sea level change, whereas within orogenic belts, climate becomes somewhat more subordinate as a driving mechanism for Pennsylvanian sea level change even though indicators of climatic change itself are preserved. Methods developed herein permit estimating magnitudes of both tectonic and glacio-eustatic components of sea-level change influencing Pennsylvanian cyclothem deposition, and may be applicable to other cyclic sequences.