Palaeoenvironmental and palaeoclimatic reconstruction of the Upper Cretaceous (late Campanian–early Maastrichtian) Horseshoe Canyon Formation, Alberta, Canada
Abstract Pedogenic features and geochemical signatures of paleosol profiles exposed in the Horseshoe Canyon Formation (HCFm) were studied to reconstruct the ancient environments and climates that prevailed at high latitude (~ 60°N) in the North American Western Interior during the late Campanian to early Maastrichtian. Eight pedotypes representing hydromorphic, moderately-drained and well-drained paleosols were recognized. When considered within their stratigraphic context, these paleosols show that palaeoenvironmental and palaeoclimatic changes occurred either gradually between major lithological subdivisions of the HCFm or relatively rapidly within subdivisions. Hydromorphic paleosols, predominant in the coal-bearing stratigraphic interval at the base of the HCFm, were gradually replaced by well-drained paleosols in the overlying coal-poor interval. While climate remained warm and humid during this change in soil drainage conditions, mean annual temperature and precipitation dropped rapidly higher in section, shortly prior to the Campanian–Maastrichtian boundary, without concomitant lithological or pedological changes. A brief return of poorly-drained paleosols in the middle of the HCFm coincides with a regional marine transgression (Drumheller Marine Tongue). Well-drained palaeoenvironments, associated with a warm and dry climate, persisted through the upper part of the HCFm until a gradual return to poorly-drained conditions and to a warm, humid climate occurred at the top of the formation. An important decline in turtle diversity through the HCFm, previously attributed to climate, is shown to coincide instead with changes in soil drainage conditions, where aridity, landscape instability, and migratory barriers possibly acted as secondary limiting factors to faunal recovery.