PALEOCEANOGRAPHY,VOL. 1, NO. 4, PAGES 403-416, DECEMBER 1986 UPWARD SHALLOWING PLATFORM CYCLES: A RESPONSE TO 2.2 BILLION YEARS OF LOW-AMPLITUDE, HIGH-FREQUENCY (MILANKOVITCH BAND) SEA LEVEL OSCILLATIONS J. P. Grotzinger Lamont-Doherty Geological Observatory, Palisades, New York Abstract. Shallow-water carbonate perhaps influencing global sea level platforms, characterized by sequences of through minor changes related to small- small-scale upward shallowing cycles, are scale continental or alpine glaciation. common in the Phanerozoic and Proterozoic It is possible, then, that Milankovitch stratigraphic record. Proterozoic small- band climatic forcing has occurred for at scale cycles are commonly 1 to 10 m thick, least the last 2.2 billion years of earth have asymmetrically arranged facies, and history. are strikingly similar to Phanerozoic platform cycles. In some platform INTRODUCTION sequences (eg. Rocknest, Wallace, and Helena formations of early to middle The stratigraphic record of geologic Proterozoic age), it can be demonstrated history is characterized by intervals of that the lateral distribution of facies pronounced rhythmic or cyclic arrangement within cycles relates to systematic of facies such that repetitive groups of variations in platform paleogeography and rock units have component facies which topography. In the Rocknest formation, tend to occur in certain order. cycles with intervals of tepees and Sedimentary cyclicity has been documented pisolitic breccia formed on a topographic in many paleoenvironmental settings high (shoal complex) near the shelf edge ranging from fluvial to deep sea, in both rim, and provide evidence for eustatic carbonate- and siliciclastic-dominated falls in sea level at the end of each systems [e.g. Beerbower, 1964; Hays et cycle. The presence of these facies in al., 1976; Olsen, 1984; Arthur et al., other Proterozoic cyclic platforms also 1984; James, 1984; Goodwin and Anderson, suggests that eustatic sea level falls may 1985]. Both autogenic and allogenic have been important in the development of models have been proposed to account for each cycle. Proterozoic upward shallowing this cyclicity, but in recent years new cycles appear to have had periods of evidence has been collected that is between 20,000 and 100,000 years, and compatible with a Milankovitch-forced probably formed during eustatic control over many cyclic systems. oscillations in sea level with amplitudes The effect of Milankovitch band forcing of less than 10 m. This suggests that on cyclicity of Pleistocene deep sea cyclicity may have been regulated by sediments is almost irrefutable [Hays et Milankovitch band climatic forcing, al., 1976], and Milankovitch band forcing is probably the cause of cyclicity in Copyright 1986 earlier Cenozoic [Mathews and Poore, by the American Geophysical Union. 1980], Cretaceous [Arthur et al., 1984], and Jurassic deep water sequences. The Paper number 6P0531. recognition of Milankovitch band climatic 0883-8305/86/006P-0531510.00 forcing in Cenozoic and Mezozoic deep 404 Grotzinger: Upward Shallowing Platform Cycles problems arise primarily from poor exposures of many cyclic platforms, which prohibits detailed studies of individual cycles on a lateral basis, in turn leading to a poor understanding of cycle dynamics and determinant mechanisms. This paper provides new insights into the general question of cyclicity of shallow-water platforms in geologic history by (1) discussing the paleogeography and cycle dynamics of a superbly exposed early Proterozoic cyclic platform and its significance in understanding general mechanisms of cyclicity, (2) discussing general models for the development of platform cycles, independent of age, and (3) reviewing the occurrence and possible causes of other cyclic sequences in the Proterozoic record, thereby establishing that cyclic platform sedimentation, and perhaps Milankovitch band forcing, has been J ROCKNEST FORMATION important for at least the last 2.2 billion years of earth history. Fig. 1. Location of Rocknest formation, Wopmay orogen, northwest Canadian Shield. MECHANISMS OF CYCLICITY, ROCKNEST PLATFORM marine sediments has been successful The early Proterozoic (1.9 Ga) Rocknest primarily becase of three factors: (1) formation is exposed in Wopmay orogen, other extrinsic controls such as episodic northwest Canada (Figure 1). It is part subsidence have a minimal effect on of a continental margin sedimentary prism stratigraphic accumulation in the deep-sea composed of a basal rift sequence, a environment, (2) the deep-sea record tends middle passive-margin sequence, and to be more complete than that of platform overlying foredeep sequence [Hoffman, sediments, and (3) biostratigraphic and 1980; Hoffman and Bowring, 1984]. The magnetostratigraphic zonation are upper part of the passive-margin sequence excellent for the Cenozoic and Mesozoic. (Rocknest formation) is a cyclic, dolomite In fact, the evidence is so overwhelming shelf sequence with a stromatolitic reefal for a Milankovitch-forced climatic control rim and flanking debris apron [Grotzinger, over Cenozoic and Mezozoic deep marine 1985, 1986a, b]. cyclicity that it has been suggested to The palinspastically restored shelf adopt the use of sedimentary cycles in sequence (Figure 2) is an eastward determining the true length of thinning prism, up to 1 km thick, chronostratigraphical zones as a new extending for over 220 km parallel to approach towards establishing an absolute depositional strike and over 200 km time scale [House, 1985]. perpendicular to strike [Grotzinger, 1985, The mechanisms responsible for 1986a, b) . The shelf sequence can be cyclicity in shallow-water marine platform divided from west to east into slope, sediments are less certain [Weller, 1964; outer shelf, shoal complex, and inner Wilkinson, 1982; James, 1984]. Although a shelf facies (Figure 3). Slope, outer Milankovitch band forcing effect has long shelf and shoal complex facies assemblages been suspected [Gilbert, 1895; Weller, are restricted to the western margin of 1930; Fischer, 1964], new evidence is the shelf; inner shelf facies occur over being gathered which supports this model most of the shelf region except adjacent [Goodwin and Anderson, 1985; Grotzinger, to its margin. Slope and outer shelf 1985, 1986b; Read et al., 1986; Heckel, facies are discussed in detail by 1986]. However, general acceptance of the Grotzinger [1985, 1986a], and are not Milankovitch model for shallow-water reviewed further here. Shoal complex and sediments has been slow because of a lack inner shelf facies are briefly reviewed of understanding of the local mechanisms here as they pertain to shelf cyclicity, which govern platform cyclicity, and how, and more detailed discussions of these if important, a Milankovitch control would facies can be found in reports by regulate platform cyclicity. These Grotzinger [1985, 1986b]. Grotzinger' Upward Shallowing Platform Cycles 405 W ROCKNEST SHELF - NORTH ß ß ß vv ß ß ß ß MNI*. %%* • IOkm r'• TUFA-BASEDCYCLES DOLOSlLTITE-BASED CYCLE .'[-•OUTER-SHELF SHALE-BASEDCYCLES SLOPE SHALE ß MEASURED SECTION TIME W ROCKNEST SHELF - SOUTH E VVVV W ß ß ß ß ß UNI•- I lOOm IO km UNI UPPER NON-CYCLIC INTERVAL "GRANDCYCLE" MNI MIDDLE LNI LOWER " Fig. 2. Palinspastic cross sections of Rocknest shelf stratigraphy. Note well developed W-E facies zonation including slope, outer shelf (rim boundstone and backreef grainstone), shoal complex (tufa-based cycles), proximal inner shelf (dolosiltite-based cycles), distal inner shelf (shale- based cycles), and lagoon (shale) . Shoal Complex_ During the course of cycle development at the shoal complex, deposition of Lithofacies. Shoal complex facies laminated dolosiltite and lutite, occur in a narrow belt, 1 to 5 km wide cryptalgalaminite, and tufa was followed (Figures 2 and 3). Sediments are arranged by lowering of the water table and cyclically, so that cryptalgalaminite, establishment of a vadose zone which tufa, and laminated dolosiltite and lutite favored tepee,breccia and pisolite are overlain by disrupted equivalents, formation. These facies are critical in developed as tepees and breccias with deriving the mechanism responsible for associated pisolite (Figure 4). cyclicity, suggesting strongly that 406 Grotzinger: UpwardShallowing Platform Cycles w ROCKNEST SHELF 5-10 km = I •- 100-200 km - I LAGOONAL FAIRWEATHER SHOAL COMPLEX .•, o MUDS (• o WAVE BASE o SANDS STORM o W. CYCLIC TIDAL FLATS BACK REEF SANDS SHELF EDGE LOWERSLOPE • REEFS UPPER SLOPE RHYTHMITES,..• RHYTHMITES,EDGE.CGL.,8• TURBIDITES,8• DOWNSLOPEBIOHERMS BRECCIAS Fig. 3. Rocknest shelf paleogeography. Cyclic deposits formed by limited westward and extensive eastward progradation of shoal complex in response to low-amplitude, high-frequency sea level oscillations (see text). eustatic sea level falls were important in kilometers) and pass westward into shoal establishing a vadose zone at the top of complex facies and eastward into lagoonal each cycle. siliciclastic shale, siltstone, and Interpretation. Cyclical sequences in sandstone. Generally, inner shelf facies the shoal complex are interpreted to occur in asymmetric, upward shallowing reflect low-amplitude, high-frequency cycles which can be classified according oscillations in sea level [Grotzinger, to cycle base lithology, reflecting 1985, 1986b]. Initially, upper intertidal paleogeographic position on the shelf to supratidal sedimentation occurred (Figure 4). Shale-based cycles have the during small rises in sea level and most diverse facies