Glacigenic Characteristics of the Luoquan Formation and Sediment Gravity Flow Reworking on It

Abstract

The origin of the Luoquan Formation which occurs along the southern margin of the North China Block has long been argued. Based on recent work, the Formation is considered as a glacial sedimentary sequence partially reworked by sediment gravity flow. The major evidence for the glacigene of Luoquan Formation dïamictites is as follows, I. a striated and polished pavement with various features resulting from glacial abrasion and plucking, such as crescentic gouge, crescentic fracture, streamlined form and glaciated step; 2.unsorted diamictites with striated clast, faceted clast and iron-shaped stone formed by glaciation; 3. rhythmite with dropstones; 4. a glacial sedimentary sequence bearing advance-retreat cycles; and 5. wide distribution of the diamictites. Glacial deposits can be distinguished from sediment gravity flow deposits by the features mentioned above. Some characteristics of sediment gravity flow existing in the Luoquan Formation diamictites indicate that glacial deposits might have been partially reworked by sediment gravity flow. Therefore, this paper suggests that the Luoquan Formation diamictite is a result of a glacial event rather than a mud flow deposit. The primary tillites are the principal contribution of the Luoquan Formation, while sediment gravity flow de.posits are the redeposited diamictites and should be termed as glacigenic sediment gravity flow deposits. on tillites.

Basal Melt‐out Structure in the Luoquan Formation and Its Significance1

“…In the column there occurs distinct ice pavement between the Luoquan Formation and the underlying quartzose sandstone cPt3s)of the Sanjiaotang Formation. Guan Baode et al (1986)and Wu Ruitang et al (1988) have discussed it in detail. According to various kinds of fracture structure and abrasive structure on the ice pavement it is inferred that the glacier in the area flowed from south to north at that time.…”

“…On the basis of previous work, Wang Yuelun et al (1980), Guan Baode et al (1986) and Wu Ruitang et al (1988 compared their depositional features with those of some recent tills in the Arc-Distributed widely on the southern margin of the North China platform, the Luoquan Formation is composed mainly of massive diamictite and bedded diamictite with small amounts of varved clay and lenticular conglomerate and sandstone, of which massive diamictite usually accounts for more than 2/3 of the total thickness of the section. Therefore, the understanding ol the processes of deposition of massive diamictite is one of the keys to the analysis of the depositional environment of the Luoquan Formation.…”

“…Since the middle and late eighties, mo-re intensive studies have been made on the origin of diamictites of the Late Precambrian Luoquan Formation of China. On the basis of previous work, Wang Yuelun et al (1980), Guan Baode et al (1986) and Wu Ruitang et al (1988Ruitang et al ( , 1991 compared their depositional features with those of some recent tills in the Arctic region (e.g. Alaska and Spitsbergen) and held that both are analogous.…”