Abstract

Isaac Channel 3 in the Castle Creek South study area exposes a leveed channel system that is up to 90 m (295 ft) thick and extends laterally for at least 1.6 km (1 mi). It overlies a thick, areally extensive, carbonate-clast-rich debrite, which is locally overlain by a thick, coarse-grained, sand-rich, sheetlike deposit. In addition to the irregular topography along the top of the debrite, the overlying sand-rich deposit helped focus subsequent flows that ultimately localized and promoted the development of Isaac Channel 3. Isaac Channel 3 comprises four channel-fill units bounded on both sides by genetically related levee deposits. Channel-fill units are made up of two or more channel fills, which, in their axes, consist typically of thick-bedded, massive to graded, pebble conglomerate and very coarse- to medium-grained sandstone (Ta Bouma or R3/S3 Lowe sequences) and mudstone-clast breccia. The uppermost channel-fill units of Isaac Channel 3 show a distinct lateral trend, in which strata become progressively more interbedded with siltstone and very fine- to fine-grained sandstone (Tcde turbidites). This trend is particularly evident toward the channel margins. Proximal-levee deposits, occurring adjacent to the (northwest) outer-bend channel margin, typically consist of fine- to medium-grained, medium- to thick-bedded Ta-c turbidites with a sandstone percentage of up to 70%. Laterally these strata thin over distances less than 300 m (984 ft) and then persist as thin-bedded, very fine- to fine-grained sandstone and siltstone Tcde turbidites. In addition, laterally extensive overbank splay units, which consist of thick, coarse-grained sandstone turbidites, are common in distal-levee deposits. However, inner-bend levees consist of mostly fine-grained, thin-bedded turbidites that also thin and fine laterally but are significantly thinner and finer grained compared to strata on the outer-bend channel margin.

As a genetically related pair, channel-fill and levee deposits are markedly different depending on which side of the channel they formed. Along the outer bend, these strata are separated by a sharply erosive, terraced surface that suggests episodic channel-levee growth and long-term migration of the entire channel system toward the northwest. In contrast, along the inner-bend margin, there is a distinct upward change in the spatial association of channel-fill and levee units. The lowermost part of the inner-bend levee is truncated by the channel margin, probably associated with the early, more erosive stage of channel development. The uppermost part of the inner-bend levee, however, interfingers with channel-fill deposits, suggesting deposition on the lower energy, inner-bend levee of a perhaps narrower, more sinuous channel.