碳酸盐岩旋回沉积的形成机制

The mechanisms for the generation of carbonate cycles

为了更好地理解碳酸盐岩旋回沉积的形成机制，计算机模拟可谓是一种既直观又定量的好方法。我们重点选择了与沉积相有关的五个重要参数：（１）与水深相关的沉积速率；（２）时滞（Ｌａｇｔｉｍｅ）；（３）盆地线性沉降速率；（４）潮差；（５）海平面振荡幅度与周期等。由于碳酸盐岩沉积物对海平面变化非常敏感，因此研究中特别注意了海平面振荡幅度与周期的变化对碳酸盐岩旋回沉积的控制和影响。一个几米厚的潮坪旋回层序通常是由海平面的低幅振荡形成的，振荡幅度也就是几米，时滞亦较短。当海平面振荡幅度较大时，在海平面下降期间，滨线向海迁移的速率会超过潮坪的侧向加积速率，因此在内陆棚上缺乏潮坪沉积物。海平面振荡幅度的增加也能造成海平面下降快于潮坪的沉降，滨线向海迁移速率比潮坪进积的速率要快得多，从而在旋回层序内发育较厚的渗流带以及不整合界面。

Computer modeling provides a quantitative approach to a better understanding of actual carbonate cyclic sequences. To model carbonate cycles, we can use water depth dependent sedimentation rate for each facies, an initial lag time, linear subsidence, tidal range, and period and amplitude of sea level oscillation about a horizontal datum. Tidal flat capped cycles up to a few metres thick result from low amplitude sea level oscillation of a few metres and short lag times. Nonerosive caps reflect sea level lowering being balanced by subsidence, and basinward migration of the shoreline not exceeding tidal flat progradation rate. When higher amplitude sea level oscillations occur, the tidal flats are abandoned on the inner shelf during sea level fall, because seaward movement of the strandline outpaces progradation rate of flats. Increased amplitude also results in sea level falling faster than flats can subside, so that disconformities with thick vadose profiles develop. High amplitude(100m or more) oscillations result in incipient drowning of platforms and juxtaposition of deep water facies against shallow water facies within cycles. Sea level falls before the platform can build to the sea level highstand, and the shoreline migrates much more rapidly than tidal flats can prograde; thus, cycles are disconformity bounded and lack tidal flat caps.