Depositional Framework of Sandy Mid-Fan Complexes of Tourelle Formation, Ordovician, Quebec

Abstract

The Lower Ordovician Tourelle Formation was deposited on a series of small coalescing submarine fans along the eastern margin of an unstable fore-deep trough during early development of the Taconic orogeny. The following features suggest deposition within the mid-fan area: (1) a high proportion of coarse, massive sandstone layers with less amounts of shale, siltstone, and classic turbidites; (2) the presence of deep sand-filled channels; (3) association with olistostromes; (4) a pronounced segregation of coarse sandstones into amalgamated packets separated by thick muddy horizons; and (5) the presence of both thinning- and thickening-upward sequences. Sand packets are characteristic of the unchanneled mid-fan lobes. Some sandstone packets have thinning- or thickening-upward trends in layer thickness. Most, however, show no systematic layer-thickness variation. Each sandstone packet represents deposition associated with development and basinward progradation of a single mid-fan lobe complex. Lobe abandonment, caused by avulsion upfan, results in accumulation of a mud blanket on top of coarse facies of the active progradational phase. At a later time, the same part of the fan may be reoccupied, allowing deposition of yet another sand packet. Sequences deposited in mid-fan channels differ from lobe sequences in that they are coarser, contain deeper channels, lack thick mud horizons, and contain a higher percentage of asymmetric cycles which thin upward. The present submarine fan model places emphasis on thickening- or thinning-upward cycles for environmental interpretation. These asymmetric cycles are, however, uncommon in the Tourelle Formation. If flow size is controlled by a combination of external factors such as rate of sediment supply, frequency of major storms or earthquakes to initiate submarine mass flows, etc, then environmental controls on layer thickness will be masked, and recognizable asymmetric cycles will not be common. External control of layer thickness trends will not, however, affect the observed stratigraphic segregation of coarse and fine facies which characterizes the depositional lobes. This development of alternating amalgamated sandstone packets and mud blankets may be a more useful criterion for recognition of ancient mid-fan lobe sequences.